My question is, what is the difference between a bottom firing sub(like stf-2) and the side firing(like axiom). i am looking for a good sub in the $500 range to fill my 24X24 room and was wondering which to get. will a side firing fill the room better? any help is greatly appreciated...

There's no real acoustic benefit of one configuration over the other. It's mostly a design decision. If you have small children and/or animals, however, you'll the extra protection a bottom-mounted driver gives you.

cocheex, amble ovr to the SVS site and look at the PB1-ISD or the 20-39 PCi. Highly recommended and should fill your space nicely. Give those folks a call and they'll dial you in.

The STF-2 or 3 would be an excellent choice as well.