There are panniers available that are easily carried around with you. You don't have to leave them on the bike all day. I'm not providing examples, because there are several, and they're easy to find. I don't really have any experience with commuting backpacks.

What were your search criteria? I don't care about starting a new thread, but it's difficult to believe this issue hasn't been discussed in two years, and there should be some good information out there for you.