I find backpacks on bikes to be uncomfortable, mostly because my helmet bumps the top of the bag when I'm in riding position, and because they limit my rear view when I turn to look behind me. I occasionally use my roadie to commute, rather than my dedicated commuter, and I carry my stuff in a messenger bag. It's more weight on one shoulder, but it doesn't bump my helmet, even when it's cinched high up on my right shoulder, and it doesn't block my line of sight when I turn my head to the left to look rearward.

I have a Crumpler Super Snipe (discontinued model) that holds a change of clothes, shoes, and a few toiletries, wallet, keys, phone, etc. It's comfy enough for a few miles, but I'm quite happy to take it off my back once I arrive! I am more than pleased with its durability and performance, and definitely recommend the brand.

I see that you're riding 28 miles....backpack is probably the way to go for weight distribution. Maybe take your helmet to the store with you and try a bunch of packs on?