Hi there,
I was evaluating the same 2 carts six months back.. I can't remember what exactly made me go with ecommercetemplates over xcart, but I have been extremely happy with my decision. I've built about 5 sites with it so far (well a few are still in development) the most finished being
www.hookmarks.com. I would recommend the PHP version if you have a choice because there are many more mods available than for ASP and there are some mods that you will definitely want (image upload for example.) Speaking of which the froogle mod is great too!
If you have any specific questions about ecommercetemplates feel free to ask, hopefully someone with xcart experience will post here too.
Good luck!