I'm currently in stage two of your predicament. Went from backpack (accompanied with the sweaty back) to single pannier.

I commute to university (38km round trip), so once I get to school I carry my bag around. Panniers are great for the ride, but a bit of a pain once I get to campus. If my load is small enough, I can shove a small backpack inside my pannier (but this gets difficult when the weather is cold, since that means thicker clothes to change into).

I think that if your commute isn't too long and you'll be running errands, then a backpack is the way to go. With the right features (ventilation, straps that you can tighten and clip together, etc), then it should be more than fine. Some backpacks are really nice...