I sometimes go in the mountains where there are no sources for water and I have tried different methods of carrying water and to me, the easiest way is a 70 oz Camelbak. I use the 100 oz for the mountain bike and put other stuff in there but on the road a 70 works better and a 50 might be better for someone who doesn't like the feeling of a pack. It really doesn't bother me and when it gets empty it's very light.
I hate having a bottle in my jersey pocket, very uncomfortable and when it's empty you still have to carry it unless you use a disposable bottle.
I've also tried a handle bar bag with 2 bottles and that's a better option than the jersey pocket but I didn't like the weight on the bars and it takes away a bit of hand room.
The behind the seat cages are called "bottle launchers" for obvious reasons. I could never use them because they would interfere with me moving back behind the saddle.