I can't say I agree with the messenger bag pluggers. I have never noticed and great difference with a stabilization strap. It might make the bag 10% less irritating. Especially when you have to get out of the saddle, having any signifigant weight in the messenger bag tends to be unwieldy.

I much prefer my small backpack that I also use for hiking. It has a sculpted back so it is comfortable even when in the drops, and it fits so close there is no danger of it 'flying up and hitting me in the back of the head.'

One downside to a backpack. VERY sweaty back.

But I also have to agree with a rack and panniers being the very best, most comfortable option.