If you are going to be riding a "road" bike, I take that to mean drop handlebars, a la racing style.
You'll need a bag that's going to stay planted firmly on your back and not swing around.
I ride a Canondale Bad Boy Ultra, which is a hybrid road bike, thin tires, upright handle bars, lefty fork.
Two days ago I took my camera, 5DII and 70-200 with absolutely nothing else, in a new Lowepro slingshot 200 which has the extra strap across the chest from the waist.
It stayed perfectly in position, was easy to swing around when I stopped to take pics, is lightweight also.
I rode for two hours, up some very steep hills. I never felt too sweaty under the bag. Yes, it got a bit tiring, but so what. That's the price you pay. I look at it as an added part of the workout.
Slingbag or backpack is your best bet.
