If you carry heavy stuff, or have an aversion to back sweat, go the pannier route.

Personally though, I'm not a pannier fan. Panniers make my bike feel heavy and less agile, so I use a backpack for my 15 miles of bike commuting. I'm a sweaty mess when I get to work, no matter what, so back sweat is no issue at all. And I try to keep my load at a reasonably sensible weight.

I initially tried brands like Osprey and Ortlieb, but my personal choice of backpack is a Chrome Barrage. I've been using it since they first came out, maybe 4 years ago? It's pretty much totally weatherproof, and ballistic as all getup. The cargo net on the back is great for storing my helmet, or other large objects when shopping.