Personal preferance. In a slow moving circle, big ones are better IMO because you can get a couple lung fulls without actually hitting the bowl more than once, and thereby taking more than your fair turn. What this means, is you get more smoke each time the bong goes around, getting you higher faster. When you have to wait 5 to 10 min to take a hit, getting the most out of your turn is important. On the other hand, big bongs are heavy, and easily dropped. Also they tend to be more expensive assuming quality of materials/style remains the same between big/small. I enjoy smaller bongs most of the time however. Can't really explain why, they just seem more relaxed. I love hitting big bongs, but if I purchase a bong it will definately be small. It's something I'm going to hit on regular basis, and using a big bong is like hitting a grav bong on a regular basis. Sure it gets you really high, but sometimes its cool to just sit back, relax, and let the high come on slower.

So there is no quick answer. I think it comes down to the needs of the person/moment and personal preferance.