How much are you planning on carrying? Either works fine. The advantage to backpacks is you can get a small one for cheap. You can get away with just a rack if you are just wanting to carry a lock and some small stuff.

Play with your setup. Take what you think you'll need. If you never use it get rid of it. Figure out what you should have brought. Hindsight is 20/20 as they say. But it helps you plan for the next one.

Here's my commuter with a rack and bag. The bag contains a pump, spare tube, bungee, gloves, zip ties and scissors. The bag in the pic was less than $2 at Goodwill. Not shown is the backpack I also carry.

[IMG]83rd Ave Bridge by res1due, on Flickr[/IMG]