I have a successful sub-domain
seo wise with the top domain having absolutely nothing to do with the content of the sub-domain.
Looking back it would be appropriate to create a sub-domain that is relative to the top level domain but that didn't affect us either way. It didn't affect the search results for our keywords.
my point is you can do what you want. in my opinion if you like the looks or the sounds of your sub-domain go for it. you can still get the exact same results your looking for either way - in my opinion.
I know a few sites that offer business listings in directories. What i sometimes see with them is;
business-listing-domain.com
and then they sub-domain (if that can be verb)
toronto.business-listing-domain.com
mississauga.business-listing-domain.com
oakville.business-listing-domain.com
hamilton.business-listing-domain.com
In your case I may create and use a sub-domain if I was going to be making several guides but I wouldn't only for one guide.
sports-betting-guide.domain.com
pool-betting-guide.domain.com
boxing-betting-guide.domain.com
Actually I believe it may work in your benefit to deliver more relevant content whenever you get the chance - from your top-level domain. In your case the betting guide seems quite relative to you top-level domain so it should be better to keep the guide up there...
that's my most recent opinion...