I like both the backpack and the panniers.
Both are put on the rear rack, so my back doesn't sweat and carry the weight. Well, except for shorter rides/errands - I keep the backpack on my back for those, as there's not a second "lost" in taking it off the bike.

Backpack is my first option - using panniers when I need more luggage space, or when there's a heavy rain forecast. I find the backpack more convenient for every day use.

With panniers: Ortlieb, hands down. Waterproof - like, really waterproof. The clothes and stuff stay dry, whatever the weather. Not having to put clothes in waterproof bags, check if they are nicely enclosed, dry the bags afterwards...
Another thing I like are the straps for easy, hands-free carrying the panniers. Plus they are easily put on/off a rack within seconds and you can carry just one. This can be a downside if you plan on leaving the panniers on the bike when parking it.

Almost ten years now with the Ortliebs, still haven't been worn/cut (knock on wood).