At first the Fuji GA645Zi seems attractive for the zoom, but then you find that its zoom is slooowww... only f/4.5 at the 55mm end, and going to f/6.9 at 90mm. That's what has stopped my interest in this otherwise neat rig. I do have the GA645Wi with the 45mm f/4 and that's about as slow a lens as I can put up with for my projects.
Also, the cool Bronica RF645's lenses are f/4 for the 45 and 65, and f/4.5 for the 100 and 135 (if you can find/afford one!). I don't use the external 45mm viewfinder, going with the whole RF window, but then having dedicated 45mm framelines (compensating for both parallax and field size) is one reason I carry the GA645Wi. I do admit it's a relief to use the Bronica for the abilty to see just where the focus is...
More lens speed and lens choices in a 645 SLR of course, but more bulk too. Even the compact Pentax 645 when fitted with the 45-80mm zoom gets pretty bulky. The zooms are generally f/4.5, but the primes are faster, mostly f/2.8 which can be useful.
Ruvy, you might like to try a Fuji GS645S or GA645 (both with the 60mm lens) and see how you like it. The GS is very small and light, and an all-manual RF camera. The GA is a giant point'n'shoot with its own charms... Or a used Bronica RF645 with the 65mm would be great too. Have fun!