If you opt for a trunk bag....
Look closely at Topeak. If you also get a compatibe Topeak rack, it mounts and dismounts VERY easily with a slide and click. Beats bothering with the four little velcro doo-dads for the universal mount. Really nice if you have to take the reack on and off daily, like where you park.
They have frame mount and seat post mount versions of the rack available. Also have a couple of different bag models, some of which have micro panniers that expand out of the otherwise pretty useless side pockets.
I was very happy with it and still use it on some rides. But I no longer use it on my commute. I got a free set of panniers, which come in handy because I occasionally lash my laptop to the top of the rack. That's hard to do with the trunk bag in the way. The trunk bag only works with little things.
So the net of my position:
-- If you insist on bagging it, messenger bag.
-- If you can, don't use a bag. Use a rack, with either bungee, trunk, pannier, suiter, etc. sized to your load.
Anything to get weight off your back, for three reasons: 1) sweat; 2) lower cg; 3) no weighty straps pulling on you.