Hi guys, giving my two cents worth of input for those considering medium format...
I've been shooting MF for over 20 yrs... have owned and used Hassy 500CM, Rollei 6008SRC, Mamiya 6, and some TLRs; now using Hassy 503CXi and 903SWC.
You'd notice all the above are 6X6 square format, and I've never attempted to go 645 or 6X7 reason being I don't want to be bothered by having to decide to shoot portrait or landscape. 6X6 is a very pleasant format to shoot with. You can compose using a square, or later decide to crop to landscape or portrait... either way, you compose, shoot and get the picture first.
Also, you'd notice that the 6X6 cameras are the most compact. A M645 is not smaller than a 6X6 Hassy. But you'd notice the Mamiya RB67 or RZ67 is an enormous camera that would be nearly impossible to shoot handheld.
Metering prism are an option on most medium format. But really, when you're shooting MF, you mostly get 12 shots per roll of film, and you're not likely to just fire away like 35mm or digital. Most MF shooting would therefore be done more thoughtfully, and more deliberatedly. So I'd use a handheld lightmeter to really get my exposure right before I shoot.
I have a metering prism on my Hassy 503CXi but it's does not have any exposure meter in it. I only use the 45 prism when my shooting angle does not permit me to use the waist level finder.
For MF shooting, keep it simple, keep in clean, and enjoy. Don't clutter yourself with all the 'gadgetry' when you're shooting MF.
Hope this helps.
br porcupine