Oh boy. Where do I start? and how do I say this in the least words?

I'll try. Let me just say this.

As you can probably tell by now, deciding on a cart, as well as other issues like web design, marketing issues, SEO, etc., etc., is at minimum a part-time job.

To try to keep it simple - A website solution is in some sorts almost like a car. Like many people, or unlike many people, whatever the case, you might do what your doing now, go around to dealers, get pricing, kick the tires and spend some time doing research for the best decision/buy at the time.
One of the main deciding factors is your budget. Plain and simple.

When you go looking each car is different. It comes with different features. A different engine (the cart and/or system core that drives it), the interior, all manual, all electric, cool guages, bigger tires, good gas mileage, but maybe comes in crappy colors. All of these attributes, both good and bad, come with it. And, of course, there is pricing. So, of course, you want the best bang for your buck in your price range.

Like mentioned before in order to determine that decision making - it's best to put your realistic goals and requirements into perspective. That way, you can then seriously look at the available cars and effectively make a decision based on it's plus minus attributes, whether you need a station wagon, and then call it a day - so you can decide on other things like getting the sports upgrade or cd player.

What's my whole point?? Good question.
In short, if you are like most small businesses upgrading is a vital step (especially if your getting boxed in - prelude to my whole point), BUT and there's a big butt - chances are you have a budget. So, therefore, as you sort of mentioned before, you DON'T have the money OR time to put into something - NOT get what you ultimately want or need, lose that money and time spent and have to REDO it again.

So what's your Point?? huh?? Oh yeah. Right. sorry. So if you only have so much time and money then this is what you need to consider:
1. Remember - Go into this knowing that what you hope to do is get a solution. Money out = viable working solution (for example a cart / site that handles the bandwidth, order flow transactions, editability, and above all - makes you money.)
2. Know your considerations / goals and needs for the site. (Do you plan on doing heavy advertising and anticipate heavy traffic? Is security a main issue? Tranaction fees? How much can I spend? What's my audience?)
2. Just like a car, you might want to go with a name brand in your case like a Honda or Toyota. Something I can get that is not uncommon, anyone can work on it, I can get parts, and something I can drive around a lot and put some miles on it. So, for example, I would consider VirtueMart or OSCommerce. It's low cost, proven, you can get parts for it, someone can work on it and you can drive it for a while. There are others in the main stream to consider - Zen Cart, AgoraCart, whatever. Stay within the mainstream is my 5 cents. You can depart from the path, but you better seriously have a good reason why, meaning one seriously good attribute you need specificly for your store. Otherwise, it doesn't make sense. Why would I get an old BMW if I want to drive around a lot, don't have any other money to fix it up and don't have spare parts. Why would that make sense? (which by the way I love old BMW's but that's not the point.)

Which brings me back to the other part of my whole point. Here's the key. If your going to own and drive a car - Your gonna need a good mechanic! Sounds simple and stupid. But it's so true. Now problem is, sometimes like with your car, FINDING a good mechanic isn't always and easy as it seems. Oh sure, there's one on every corner - BUT seriously a good one - I mean a really good one - that's a different story. Oh. and I forgot. What you REALLY need is an HONEST mechanic. Yep. That's right. But just like with cars. If you find an honest mechanic your half-way there and sleeping better at night. If I get a off-breed custom cart solution (a ferarri,) You'd better have the cell phone number to a damn good ferrari mechanic. What you'd drive it around without a good mechanic for that? No. Not me. I'd drive it around looking for a good mechanic maybe. On top of that it helps if you find someone who knows something about marketing, can handle hosting and number of different other issues. Because it's not just the software package that runs itself and determines your fate, it's the mechanic that knows how to change out the fuel lines and put ties on the hoses to prevent problems from happening right before you take it for a drive to Vegas even though you didn't ask and they didn't charge you for the labor. Get my drift?
(which by the way, a all-inclusive package like Yahoo or Monster (which ain't all that inclusive by the way) just won't do that for ya.) But you will spend some time on the phone though. Which brings me FINALLY uP To the Point That you Lead INTO in the First Place:

What do I do - LEASE or OWN???
:-)
Still trying to keep it simple here.
You see. Just like a car I guess - what your talking about and no body has mentioned yet... is. You have been talking about Apples and Oranges.
One set is Yahoo and Monster, which are both all-inclusive software "SERVICES"
YOU DONT OWN THEM. YOU LEASE THEIR SERVICE under THEIR TERMS and CONDITIONS and maybe not even that, I would have to look, you just own the CONTENT.
Although there is nothing wrong with that. Because in CERTAIN situations DEPENDING on your REQUIREMENTS - THATS ACTUALLY A BETTER WAY TO GO. It serves its purpose. Nothing bad against it. BUT YOU BETTER CONSIDER IT BEFORE going in. I dont own Google's GMail I just use it.
VERSUS -
The flip set to that is OWNING your OWN STORE. Meaning nobody really owns exclusively the software (BUT THE CUSTOMIZATION YOU DO - See above about mechanic) BUT you do MAINTAIN, RUN and MANAGE your own copy of your own software on your server.

For me its easy. If your a small business. Your making money. Your on a budget. You need to take your company to the next level. You need to retain a strong online presence and that business is so important that your willing to "risk" managing it yourself, then you need your own Website on your own managed server running your own system. Flat out.
Note that there is nothing wrong with Monster or Yahoo other than the above mentioned and they're are major benefits in choosing them. This is especially true if you dont need major customization, are o.k. with there "service" package, need something up fast, and only expecting certain volume or just plain down-right dont want to deal with it, then a service like Yahoo or Monster might just be better. But if your a small business your in retail, you have future plans to be in business online, you can handle the responsibility, and you need control over things like who you choose to process your payments and what you pay then you need a sole solution.

SO. AFTER all that. Short version is You might want to consider a main popular low-cost cart solution (Virtue, OSCommerce, ZenCart, etc.) running on your own owned server space, budget for that and factor in money to customize it.

And let me point this one last thing out. Just like with you new car - It's just not going to have everything. Well, maybe. But what I mean is it's a package. What your ultimately hoping for is to get the closest you can with all that you want with all of the benefits and stay the farthest away from all the bad things or drawbacks and above all - Try at best not to box yourself into a situation, an incompatible corner, or something you cant use anymore. Time and Time I have heard stories about the horror project that started the money spent and bottom-line of no end solution or any better off after months of frustration and getting ripped off. All aside though, If you spell out your plans, requirements and budget then finding a "solution" get's narrowed down fairly quickly. And if you find the right person that "knows" what I am talking about 'here' and can implement it, then you are good to go.

Bottom-line: Given the fact that you want to maintain and keep your branding, work on or with SEO, have control over your online business, be able to fix problems, find a solution that will work for at least the next 2-3 years, handle volume, (important) Have room to grow, be stable, and make you money back from your ROI, then stay with something proven, server-side and get a good mechanic. Get AT MINIMUM medium rate hosting with decent bandwidth at a good rate. Doesnt have to cost you an arm and leg. Just beware there are a lot of bad hosting companies. Middle rate or above. Email if you want suggestions.

Oh and by the way, I can work on it and be that good mechanic for you, even though I get busy these days, only - I happen to know Caroline - - and honestly, shes a better mechanic than me. Don't let her fool you. (she's good. I'd send my Mother over there with her '87 Oldsmobile if I had to, shes that good.)

Good luck :-) Still confused?? Not to worry, sometimes it even boggles me. Back to work.