Being tall with large feet panniers that don't interfere with my heals are pretty tiny for storage.

The only time I regret this is on hot days, which is an issue in places like Texas.

But I live in a place where it has only been above 100F once in recorded history and we have really really bad pavement. bunny hopping potholes with panniers is beyond my skill set.

But I would say that while backpacks are nice a messenger bag is even better. It is nice to be able to get into the contents without taking the bag off. There are several types which have straps that are quite useful for heavy loads. I can fit an entire shopping basket of food from the grocery store in mine and that is the only way groceries actually get into my house these days.

But as to which is better I would say neither. They solve different problems for different people,