Also depends if you need them to be water proof, need a top rack bag also or just panniers, easy to remove for security, must stay on 24/7, and what is the max capacity you need for worst case.
I also have an Osprey commuter backpack when I work commute. I would go for panniers with more capacity than less. There were times when I needed my commuter backpack, top rack bag, and both side panniers to work commute in winter because it would be in the 20s in the morning and 45-55 degrees by afternoon. Needed the extra space to store 2 sets of riding gear for the temp swing along with my lunch, work cloths, and extra stuff for flats.