I always say to do what you're used to doing in training. I carry a water bottle with me when I train and I do during an event too--unless it's a 10k or shorter. But the longer runs, to me, it's more about completing the distance than it is about a winning time--so I prefer to carry water, gel, and whatever else I need to have a good running experience. Plus, I always have a hard time trying to drink out of those little water cups they hand out on the course. It's much easier to drink from a water bottle or camelback.