I roll with an Xtracycle, carries more than I could ever fit in panniers. I also have a Burley flatbed trailer, for when I need to carry something that doesn't fit well on the X--such as my wife's new microwave.

I did one ride, one time with a backpack, NEVER again, it wasn't a whole lot of fun.

If I didn't have the X, I would be solidly in the rack and pannier crowd, I would also have a trailer.

As far as convenience goes, not having to strategically pack your stuff, the X wins again---Just toss what ever you want in the Freeloaders and away you go.