I use whatever suits my particular riding need at the time. For short training rides, I go with bottles only. Once I get up in the 40 mile range, I add a 70 oz Camelbak. For anything beyond about 50, I go with my 100oz Camelbak. Sometimes I'll even use the 100oz on a 40 mile ride if I know that water availability is limited. Point is, it depends on what I know about the weather, the route, my pre-ride hydration level, and so on. I have no qualms whatsoever about carrying both if that's what I need.

I also seem to have a need for more liquid than most folks. I operate on only one kidney, which I think may have something to do with it. Two 24oz bottles just isn't enough for me most of the time.

Also, on 50+ mile rides, I like to carry water in the CB and some sort of electrolyte drink (Gatorade, Accelerade, etc...) in my bottles. I can't take that strategy if I go bottles-only.