Do you like square images? Do you prefer the shape of 6x7? Although you CAN crop anything into anything you'll always be happier shooting a format you 'see in.' Assuming you wet print, do you already have an enlarger that's multi-format, or will your camera choice have consequences in the darkroom. Small issues- 6x6 is easy to contact print and file, 6x7 is a PITA. If you can begin with a clear idea of what you want to shoot, it will help lead you to the right camera.