I've flown the backseat of one rocket and many RV-8's. I'm 6' 2" and 230lbs.
I didn't notice any difference in comfort between them. Both were pretty difficult to get out of without pulling on parts of the airplane that bend or break.
Honestly though, I think my biggest comfort problem is being 230 lbs and sitting so far off the CG. You can really throw the plane around (and be thrown around) with your weight and it often results in rides that turn me GREEN.

(or blue)
I'm sure both are fine if you're a small person. But for someone my size, it would be hard to ride back there for too long.
Phil