I have owed all three of the Pentax 645 series and a good buddy of mine owns a Pentax 6x7.
I used to shoot the NII and had an N as backup (not that I ever needed backup). I found the cameras very reliable. The N2II has a mirror lock-up and a textured finish. The mirror lock-up is unnecessary due to the great job that Pentax does of dampening the mirror. The N is prettier in my opinion but the textured finish of the NII does hide/prevent scratches.
I am a small time Ebay Seller of camera gear to help support my hobby. I have owned and sold quite a few original manual focus 645's. Like most people, I hated the buttons of the original camera. Imagine having to keep pushing a button to get the shutter speed you want. I also found the viewfinder dimmer than the N and NII.
My advice is to buy an N or NII over the older original camera even if you just use the manual focus lenses.
Comparing a 645 to a 6x7 is kind of like comparing apples to oranges. The 645 system is easier to carry and the N and NII have auto focus and matrix metering. The 6x7 is big and clunky but has that beautiful 6x7 negative. I used to shoot a Mamiya RZ67. It was a great camera but I didn't carry it around everywhere. You need to hold a 6x7 in your hands to see what you think.