I started out with a seatpost rack, and absolutely hated it. The weight is too high, the rack is hard to secure, and you can't carry much weight. I was happy the day it broke and didn't damage me or the bike in the process.

The Topeak rack is great for commuting, especially when combined with a Topeak bag designed to slide on and off. Excellent durability and easy to install with disc brakes. You need to get the disc-specific model.

Recently purchased OMM Sherpa racks for touring, and they are very rugged, but probably more than you need for commuting, and more expensive.