I would think that one big difference is that generally with the Hassy you will be using a waist level finder whereas with the Mamiya you will be using it eye level. Of course you can get a eye-level finder for the Hassy but due to the weight and shape it is quite a different experience than with the Mamiya. Just a thought.
One more thing... you are changing two things at once... camera format AND stepping into flash. If it were me I'd experiment with flash before buying another camera to make sure just how important that may (or may not) be to your new format descision. Both are great cameras, but very different.