When playing pub gigs on bass I always prefer a 3-piece. There's more room for expression in the music, more room for the band in whatever space we've been given by the door to the Gents' Toilet, and there's no need to discuss who's going to set up where - it's blindingly obvious where each of us is going to go. That's less of an issue at club gigs of course, since there's usually some sort of stage.
When playing keys it's pretty much inevitable that I'll be playing in a 4- or 5-piece, but then my role is entirely different, plus I set up at the back alongside the drummer.
I always assumed that it would be easier to get gigs for a 3-piece but in fact my 5-piece soul band seems to have no trouble bringing them in ... it's just that there's less money for each of us. Luckily enough, I don't do this for the money.