R Leo says it right when he says it is a relative thing. We're all built a little differently and, while one might find them comfortable, someone else is miserable.

As far as leather versus MBTex, I have one of each and other than surface softness, they're comparable. If yours are sagging, you might consider a rebuild with repairs to the springs and new pads.

Second thought: I used to get a tired back on long trips until I thought to remove my wallet from my left rear pocket. (No, it wasn't lumpy from being full of money.....)

Cheers,
Wes