Wonder if this depends on what part of the world/region you're in. It would be hard for me say what most people use based on what I see. If I had to pick, I'd lean towards backpack. That's not a recommendation. I think people use them because that's what they have. I also see plenty of panniers too but I see as many with two as with one and overall I'd say panniers are still in the minority.

I couldn't get by with one pannier unless it was pretty huge. Combined the panniers I have are about 2600 cubic inches but I feel like I can't get as much in them as I can my 2100 cubic inch messenger bag.