I prefer side because they keep the weight lower and allow me to more easily swing a leg over.
When I tour I need extra space, so I end up with both.

I love my panniers on the Z1000SX/Ninja 1000, they integrate seamlessly into the rear handles so you can’t tell the bike can carry bags unless they’re on. They can fit a helmet and they look nice. Starter key of course also opens the bags and locks them to the bike.

I had a top box on an older Honda, terrible experience. Taking the bag off the bike meant working with the same mechanism that held the bag shut. One time going to a date of course resulted in opening the bag while taking it off and spilling the contents in front of her.