Jim,
I have not used either of these but they were on my list when looking into 6 x 4.5 for Stevie. On 35mm she was carrying two FM bodies as essentially rather expensive film magazines. The absolute biggest downside from our perspective with the Mamiya 645 was the lack of interchangeable magazines. This feature became available on the 645 Super. If carrying various film stock is not an issue for you then that part does not apply. In our case it was important so I elected for the Bronica ETRS for her. That decision cost me a mint black Nikon F2A as a straight swap for her brand new ETRS/75mm lens/Waist level finder/Magazine and Speedgrip
. The price we pay for keeping the other half on side.
First thing to think about with these toys is metering. Standard cameras with waist level finders have no built in light metering. For that you will have to add a handheld meter or lots more expensive a metering prism finder.
The next thing you really need to consider is that to use one of these in vertical format you will need a prism finder metered or not. As you are thinking portraits this will be your biggest consideration. After all if you crop a vertical format out of a horizontal 6 x 4.5 frame you gain little in film area over using 35mm.
Next up will be the handling in shooting vertical format. With the Bronica that was solved by the Speedgrip attatchment, an all in winder and shutter release. Mamiya did a handgrip mounted on the left of the camera and would release the shutter but winding on had to be done by the knob on the right side of the body.
Given your intended subject matter for this format I would strongly recommend you having a hands on tryout. I can recommend the Bronica ETRS + speedgrip in that situation though.
Now I'll throw a curve and that is 6 x 6.
Costs a bit more to feed, but you only use the camera in one orientation and you can compose for Square, 6 x 4.5 horizontal, and 6 x 4.5 vertical, and no requirement for an expensive prism finder, but then you would need a handheld meter. Naturally if the camera is an SLR it is a bit bigger than 6 x 4.5 though. I used a Bronica SQA for years as it, and all the bits and bobs like magazines were a fraction of Hasselblad prices. In 6 x 6 you have lots of other options if you include twin lens reflex - less bulk/less weight, that is unless you want interchangeble lenses ( Mamiya C330 apart ) and detachable film magazines.
6 x 6 is probably as large as you can go handheld. When you get into the realm of 6 x7 image quality takes another leap but the cameras are huge and heavy. Thinking Mamiya RB67 and RZ67.
Well after that speil what was my take on the Mamiya 645 ....................... Bronica ETRS is far more versatile.
Don