Backpack.

I tried panniers, and in spite of mounting with two quick velcro attachments and quick-pull up straps, it's still a pain.

That backpack can make my back sweaty, but I'm going to shower when I get there anyway.

As far as a pannier lowering my center-of-gravity, I don't think 5-10 lbs on my back versus over my rear wheel is going to change anything compared to the extra 20lbs around my waist that needs to be trimmed....