I have a 13 mile commute which I feel is too long for a backpack - especially in the summer. I use a small seatpost-mounted trunk. I keep my pump in there and have my rear light attached to it. The rest of my stuff goes into a Target or other plastic grocery bag which then fits into the trunk. I leave jeans and shoes at work and just swap them out as needed, usually getting my wife to transport stuff if/when she is in the area. I also leave my cable lock outside at the office building. Then I only have to carry a shirt, other clothing necessities, wallet, work badge, phone, keys, and a few snacks.

I'm thinking of getting a slightly bigger trunk bag because sometimes it is a squeeze to fit it all, especially in winter where I may have layers to remove. I like the trunk because it seems to be the most aerodynamic and it serves as a rear fender in rain.

I'll use a small backpack once every couple of weeks to transport clothes or anything bigger. This isn't a great pic of the trunk, but it's what I had handy.