I'll usually alternate, drinking the juice one time and water the next. It's also possible to drink, say, 3 swallows of goo followed by 3 swallows of water (from the other bottle). The latter options lets you rinse your mouth, and may promote better dental health by diluting the citric acid most drinks add. Some people dilute electrolyte drinks by 50% and put that in both bottles, but it tastes ghastly to me.
If you're slowing toward the end of a 50 mile ride, though, you may need some Xade for sugar and energy. After a couple of hours you've probably burned up most of the glycogen stored in your muscles, so you're limited by how quickly your body can pull sugar out of fat and dump that into your bloodstream.