Bottles for several reasons:
1. I want to keep the weight low, in normal riding position your back is on top of everthing.
2. I want my back to be a heat sink.
3. I carry both water and Gookinaid.
4. It's generally easy to refill water bottles on the road, so no need to start with lots of water as with mountain biking.
5. Respect from other roadies, got to keep that one in mind.