There is a difference. The Mamiya 6 is more compact with its retractable optics and you don't need a finder for the 50mm. Also, there are only three optics available (50/75/150) and these fit with a body into a very small Lowepro Nova 1 bag, making this an excellent combo for street and travel.
Originally Posted by JMoore
The Mamiya 7 is a little larger, has a larger neg and the excellent 43mm lens. This lens was the reason for me to buy a 7 additional to my two Mamiya 6 bodies. After 5 years of usage, I find that I expose about 5 films in the Mamiya 6 for every film in the 7... even while I got nearly the complete range of optics for the 7... the square neg is just better for me, no problems with portrait and landscape orientation etc.