I have never been to Fish World, as I've heard from a friend that their fish are not always very healthy. I've had good luck with the Fish Store, but have yet to look into plants (just started my research on planted tanks actually!)
Surprisingly enough, I've had good luck with healthy fish from both Petcos, although the people on up O Street are more knowledgeable (rare, but I know one of the people who works in the fish dept, and has dedicated more room to his fish in his tiny little apartment than living room for himself, haha). The people on 56th are actually pretty ignorant, and it makes me very sad that I have to be 18 to even apply to work there. Lets just say that I race to the customers in the back before the employees can get to them, since I've caught one employee telling a customer that oscars were "peaceful community fish"
I'm definately not suggesting Petco over the Fish World or the Fish Store, but stating my experiences - and warnings! haha... definately NOT the place to go if you are a beginner in the fish hobby... it will turn you off from the hobby, and that would just be sad.