What do you shoot?
Certainly, for street/travel/casual photography, the Mamiya 6 is more practical. But, what does the 6 offer you that you're not getting from the Fuji, aside from The Square? If you were seriously considering the RZ, i would think the square wasn't really the primary selling point.
If you shoot people/portraiture, the RZ is a FAR better choice, as the lenses give you Actual Bokeh! And amazing bokeh at that. The 6 i had, with the 75mm was pretty awful for me when shooting people. Limited bokeh, and bad bokeh when you could get it.
For landscapes, the RZ's image size is much more of a significant increase versus 6x6 from your 645.
Sorry to be late to this. I was debating yesterday, whether to answer.... Re: the RZ versus RZ Pro II... i chose the Pro II, because of the additional fine focus adjustment knob. But, i was going to say that $350 for that kit is a killer price.
Re: the RB being 'more durable' than the RZ — i dunno. RZs were probably the most often-used cameras (along with Pentax 67s) in fashion photography, by people who shot tons of rolls per day, and logged thousands of travel miles with them. Fashion guys burn more medium format film than anyone. They were always known for being robust. I won't compare them with the RB, but i have never heard of a reliability problem with the RZ. They're workhorses.