Similar to others, I don't think putting a lot of weight on your back makes much sense for a road tour. I have used a hydration backpack, and I find it works quite well, but I wouldn't want to pile a bunch of heavy stuff into it. A lumbar hydration pack with sufficient capacity might work better, though I can't say from experience.

The only road exception is if you're trying to go über light, and decide to skip on panniers in favor of, say, a backpack and a water-proof stuff sack. But afaik that's when you are in "cut the handle off the toothbrush" territory.