I would have to say that smaller pockets will give you the most bang for your buck. However, there is something to be said for smaller balls and bigger tables, too. I play snooker and pool at home (two tables) and in my opinion there is no big transition that has to be made to play well with either set of balls. When switching from big balls/large pockets to big balls/small pockets, you definitely have to pay a lot more attention to your aim. Same with small balls/small pockets. However, I don't feel that it "undoes" the practice that I do on the pool table when I switch to the snooker table.
I play in a fairly tough tournament on Monday nights. I try to get home and practice 8-ball on the snooker table with the big balls before I go as I feel it makes shooting on the smaller tables at the tournament much easier. Shots don't look as long and tough as they used to. Ideally, you would like to keep shooting as it is easier to stay in stroke when you're playing than when you are sitting and waiting. Unfortunately, that's not the nature of the beast so I feel it is in my best interest to do whatever I can to give myself an advantage. Even a few games before I go gives me a boost of confidence that I probably wouldn't normally have.
Of course, I agree with everyone above about the differents size balls. Even playing with a cue ball that is barely larger and heavier than the object balls totally screws up your game. You can pretty much forget about cutting a ball frozen to the rail and delicate safety plays.