I can't speak for the GP, but since it has been 11 hours and he haven't answered yet I'll give you my answer: it's mostly a matter of taste, or sometimes if there is a particular library/app/framework for one which you'd like to use.

I would also add that at least in my experience the Scheme community was much friendlier the the Common Lisp one. They don't particularly like novices over there and it is quite easy to get flamed if you're not careful enough...

There are multiple CL communities. comp.lang.lisp famously sucks, but (for instance) #lisp isn't so bad.