Like all things it really comes down to how your going to use it and which suits your personal style better.
The Mamiya is a more flexible tool that can accomplish a greater range of photographic tasks but the Rollei truly shines at photographing people.
The lack of interchangeable lenses is a limitation on the Rollei, but it keeps me focused on the shooting and from messing about with my gear.
Size: I've always been impressed with the Mamiya's but just found them too big for toting around casually. Both of the attached shots wouldn't have happened if I owned a Mamiya TLR instead of a Rollei... Cause I would have left it at home that day!
and of course.. people just like my RolleiCord. It almost never fails to impress.
I hope this helps in your decision,