I have three different sets of Mamiya equipment - Mamiya 645 Pro SLR for 6 x 4.5, C330 TLR for 6 x 6, RB67 SLR for 6 x 7 (or 6 x 4.5 if I use those backs).
If pressed, I could basically do everything I need to do with any one of the three.
There are differences between the three though that make each set slightly better for certain things.
If you would like an SLR, try 6 x 4.5. If you shop carefully, most likely you can get most of your money back on a re-sale.
Each system has some complexities though, so read up before you purchase. As an example, the Mamiya 645 have a number of different models, and the feature sets vary a bit.