+1 on the Timbuk2 pannier messanger bag combo. On hot days I use it as a pannier. On cold days or when I take my track bike (like today), I use it as a messanger bag. Goes on and off the bike easily.

http://www.timbuk2.com/tb2/products/...nier-messenger

I tried the Topeak MTX trunk rack, but storage was awkward (not much protection for a laptop in one of the zipdown side bags, and it did not come off the bike as easily as a pannier and was awkward to walk around with.

The shift messanger bag / pannier works fine either way.

Its nice to have options. Enjoy whatever you choose!