My progression was M645, MAT-124G, RB67, Fuji GW690, Tower 6x6 folder, GA645, and Mamiya 7. I still have them all. I have built out each of the system cameras a bit. For the single-lens models, I have a collections of filters, mostly for B&W, and a lens hood. If I had to keep one, it would by the M645, specifically the 645 Pro TL. For me, it is the best compromise between size and weight, lens availability, and negative size. If I had to state a preference, it would have to be TTL over RF. I will say that I view single-lens RF as roughly being the same as TTL with respect to focusing - WYSIWYG. But, with TTL, things like depth of field preview and polarizer usage become possible or significantly easier to use.
Here are some notes on each:
M645 - Great system with lots of lenses, finders, backs, handles, winders, focusing screens, etc. It's easy to build the camera that you want, be it an SLR-type rig or TLR-ish WLF rig. The glass is great and using the system feels very natural to me. It isn't too heavy (for me). I love the versatility. I also love socket sets and other accessory-laden things.
MAT-124G - Fairly compact and light. It attracts attention because it is plenty weird relative to what most people 50 and under are used to. It's easy to focus and fun to shoot. I like the results from the lens, but would describe is as soft relative to most others that I mention here.
RB-67 - The beast. Similar to the M645 as a system camera in terms of lenses and accessories. I only have a WLF for this camera, so I can't speak to it's use with a different finder. I've carried this up Humpack mountain in VA as well as around the city. I like to use it for still-lifes in the house and don't venture out with it anymore due to its size and weight. For what I do with the negatives, it's not worth lugging around the weight Still, I enjoy the slowness and contemplation that the RB afforts. I've seen a guy shooting an outdoor concert with one, so don't let the weight scare you if you commit to it.
GW690 - Because 6X9. The negatives are just huge. The lens is sharp. But, you only get 8 shots per roll and there is just the one lens. Still, it is on the light side (relatively) and is fun to shoot. I've thought about doing contact prints with this, but haven't. Light meter required.
Tower 6x6 folder - I didn't pay much for it, and it is TINY!. And light. It's basically stamped out of sheet metal. It won't survive much of a drop, but you can easily fit it into a small pocket of a back pack. I'd put the lens in about the same class as the MAT, maybe a little softer. It's fun to shoot but has limited shutter speeds, so you have to choose your film for the conditions more carefully that you would for the M645. Shooting 400 speed film in bright sun, you can forget about shooting wide open (1.9 or 2.8, I forget). I took this on a international trip and it was fun to shoot. Oh, you will need to a light meter unless you have calibrated eyes, which I do not.
GA645 - Probably the most fun camera of the bunch. A really nice point and shoot with a build-in flash and auto focus. It has manual exposure plus full auto and aperture priority. The lens is awesome, but single focal length. This would be my second pick over the M645, mostly due to size and weight. This camera is much bigger than the Tower but seems like a toy compared to the RB. I've taken the most travel picture with this camera because of the size and weight. It's probably about the size of the GW690 but lighter, and 15 shots per roll (some have 16) vs 8.
Mamiya 7 - Easily the 'nicest' camera I own. This camera handles like a large point and shoot with manual focus. The lenses are awesome. The RF patch is bright making focusing easy. This camera wins over the RB simply due to weight but loses out to the M645 due to being RF instead of TTL focusing and the lower amount of bits and bobs available relative to the M645. Admittedly that is my limitation as a shooter (RF vs TTL) and I understand that others would make the opposite choice. I've been too chicken to take it on international trips (theft, breakage), plus 10 shots per roll means I spend more time reloading and less time shooting.
So, final words of wisdom (or lack thereof): Think less about the negative size and more about RF vs TTL as well as single-lens vs system-type cameras. Weight might also be a factor. If you are comfortable with RF, the Bronica RF645 or Mamiya 7 might be great for you as they offer a number of different fixed focal length lenses from which to choose and are among the lighter of the systems I have mentioned here. If TTL is more to your liking, then the Hassy or Mamiya M645 or RB/Z might fit the bill. If the need to change film types mid-roll is important to you, look to the Hassy, M645, RB67, or similar type systems from Bronica among others. The MAT and Tower have been fun cameras, but I probably wouldn't recommend them as first MF cameras.
Good luck with your decision!