I shoot 4x5 (Chamonix) and the Mamiya 7. I would say with fine grained film like Acros the Mamiya is about 90% of 4x5 for image sharpness/quality. I really don't think I could tell them apart at 16x20 size in a blind test. So I think the real reason to use the 4x5 is for the movements. A 4x5 with a wide range of movements in much more flexible, but takes longer to setup, weighs more and requires a tripod. The Mamiya is easily hand holdable in good light, quick to focus and easy to carry. I pick the 4x5 for a trip, unless I think the Mamiya will enable me to take get the camera to the shots, or the speed of setup will be beneficial.
But neither is perfect. I find the Hasselblad to be the best compromise when I'm taking portraits, so get one of them too