You have to figure it out for yourself.
I started out with panniers. Then for a while i was strapping my briefcase to the top of the rack. When I started riding a folding bike I moved to a messenger bag, and have stuck with that for ten years or so, even though I don't usually ride the folding bike now.
+1
I had this dilemma for years. I would use panniers in hot weather and backpacks in cool weather.
Then I started using a messenger bag (with cross strap). Works better than either.
- Panniers increase wind drag - last thing I want on a windy day - and my commute is fast and long
- Panniers make the bike kind of clumsy handling (unsprung weight)
- backpacks put the weight too high, and cause too much sweat.
Messenger bag sits low on my pelvis, doesn't slow the bike handling down, doesn't cause sweat on my back, doesn't increase wind resistance, really has no down sides on my commute.