The lenses on the Mamiya 7 are slower than lenses for the Leica, so low light photography is harder to do with the Mamiya. You can use 220 format film with the Mamiya, though it can be a little hard on the cameras film forwarding mechanisms, there are a wider variety of films in 120 format. So, in other words, you'll be changing out the film a lot more. And it's a little cumbersome to change it on the Mamiya 7, though with practice it get's easier and more fluid.

That said... your negatives will be stunning! Not that Leica negs aren't, mind you. It's a different, slower way to work.