There are plenty of times in the field when I wonder why I'm carrying the RB instead of one of my Nikons, then I slide an FP4 negative from the RB into the enlarger and make an 11x14 print and I remember.
The RB isn't a tool for every shot, but that big negative sure makes sweet prints.
A Mamiya 645 is a nice compromise, a nice bump up in size from 35mm but in a more manageable package than most 6x7's.
I wouldn't recommend that anyone move from 35mm/digital directly to the world of an RB67 unless they at least get to visit the world of an RB67 before deciding to make it permanent.
The OP will probably end up wanting both 645 and 6x7 anyways .
I also thought long and hard about a 67 v 645. In the end, I went with 645 and i figure if I want larger negatives, I'll go large format. Honestly, for me, the 645 negatives are plenty big. I'm happy with my system and don't see a need to upgrade in my five year future.
Bronica ETRSi or Mamiya are good choices. As for which is better, well either are on the same playing field in quality. I prefered the ETRSi because of price n availability... Also lenses are cheaper n plentyful as well as a wide assortment of accessories.
As for 6x7... Hahaha you won't realize the difference till you own one! 645 will give you very pleasing prints n you'll be amazed everytime you print.
BUT.... that very first time you print a 6x7... You will be blown away!... I mean right out of the water!
But then the very first time you print a 4x5... Holy crap!
See the rollercoaster ride in your future? Where does it end?... It ends when the wife cuts up your charge cards!!!
Ask me how i know?
Btw, get that meter and don't spare on expense! It is your main crutch n will make your life that much easier.
You know what, somehow I always missed seeing that. I only ever saw the flash sync port. Learn something new every day. I always wondered why a studio camera would need a grip to use a cable release.
The Mamiya 645 system is excellent, but I sold my Pro TL after about a year of use. I much prefer leaf shutter lenses because they flash sync at all speeds. That's why I'd recommend the Bronica ETRSi.