If forced to choose, a backpack not a messenger bag. I have hybrid bikes.

Small framed folks like me, just have less mass for a messenger bag to rest. I genuinely have a small back...my bra size is an example (circumference, not cup). I would be nearly embarrassed to say how narrow I am....

However I dislike anything on my back when I cycle. Pannier(s) ...the issue of aerodynamic is abit of a misnomer..when the pannier is near empty...for a good part of the ride.

As for weight balance, it's ok if balanced between 2 panniers. Add on front-loading and that's quite different.

I actually find some weight in my pannier(s) keeps me more steady on the bike, when it gets quite windy or when backdraft of transport trucks hit me. Well think about it, if a person is 100 lbs. and there is high winds...