Originally posted by tuco You want a "nice big negative". Arguably the 6x4.5 frame on a 120 roll is the APS-C of medium format film.
That depends whether you crop (frequently) or not. If you crop 6x6, you end up with something approaching 6x4.5, so only 6x7 gives a real advantage.
On the other hand, the square format is more attractive than I thought (I've found a nice Rolleiflex 2.8F), and I tend not to crop. I guess years of shooting transparencies does that to you.
Even so, the difference between 24x36 to 6x4.5 is bigger than that between 6x4.5 and 6x7.
Prices on 645 lenses have come down, btw, this may be a good time to buy into it—if the 645DII is in the works, as it appears to be, prices may go up again later.