I *hate* riding with a backpack on. It's uncomfortable. And it can obstruct your view when you turn your head to look over your shoulder, which is necessary to ride safely in traffic.

On the other hand, panniers are attached to the bike when you ride. You're not carrying any weight on your shoulders or torso. You're not going to get a sweaty back. Mobility and sight are unimpeded. When you get off the bike, just take the pannier with you.

There are even panniers that can be worn as backpacks, making them even more convenient off the bike. Banjo Brothers and Blackburn offer some reasonably priced models (below). Arkel and Ortlieb offer some higher-end versions.

Banjo Brothers Convertible Backpack Pannier
Blackburn Wayside Backpack & Pannier