What's your normal print size? That's an important consideration.
I have worked with both 645 (Mamiya) and 6x7 (Mamiya and Pentax) and in my opinion 6x7 is a lot closer to 4x5 than 645 is to 6x7.
Even in 11x14 the difference is noticeable, not that 11x14s from a (really good) 645 negative are all that bad.
I once had a client change the specs on a location shoot at the last minute from wanting a series of 16x20s to wanting some 16x20s and a 5x7 (feet!) enlargement. I had left my 4x5 at home (200 miles away) and only had my RB67, called my lab, they said no sweat, they made the print and it was beautiful. Happy client and one totally amazed photographer. And this was on Kodak CPS!!!