"Not only do you deal with the increased shutter+mirror shake of a 6x4.5 compared to a 35mm..."
FYI, Bronica 645 SLRs have leaf shutter lenses and decent mirror dampening which cut down considerably the likelihood of softness from the shakes. Can't say the same for the Mamiya 645.
When I was unable to replace my worn out 220 bodies because Mamiya had discontinued them, I was forced to go through a decision-making process similar to yours.
I say similar because I had to decide which medium format to use with my 35mm system. You, on the other hand, are trying to decide if you should use a 645cm system or a 35mm system. If I had to make that decision, I would have to select the 645 system because I have never been satisfied with 35mm black & white image quality.
I took a hard look at the Mamiya and Pentax 645. I had used Mamiya TLR cameras and Pentax 35mm cameras and was very satisfied with both brands. After I talked with an Alaskan landscape photographer who did a lot of great work under very harsh conditions with a pair of Pentax 645 bodies, I was convinced that the lower-priced Pentax would meet my needs. However, if price were no object, I would select the Mamiya 645.
Bottom line, if I were in your position, I would select the 645 over 35mm and I would select the Mamiya over the Pentax.
However, for my actual position, I decided on 6x7 and 6x9cm rangefinders instead of SLRs for the following reasons:
1. I liked the 6x6cm image of my Mamiya 220 and preferred the larger 6x7 and 6x9cm image instead of the smaller 6x4.5cm image of 645 camera.
2. Medium format rangefinders are quieter than medium format SLRs.
3. It is easy to focus a rangefinder with a dark filter on the lens compared to an SLR with a dark filter over the lens.
4. Rangefinders have less vibration during exposure than SLRs.
It all depends on what you want to do with the medium format camera. I have shot the Pentax PZ-1p, the Canon 1V, the Nikon F100, and the Pentax 645Nii medium format. From a size perspective, using typical lenses, they are quite similar while you are actually using them. Obviously, packing them in the bag is different but when you are out taking pictures there is not a lot to differentiate them. Ergonomically I preferred the Pentax 645Ni and ended up selling the Canon and the Nikon after about a year. (I still have the PZ-1p, but for different reasons.) I can handle the 645 handheld very easily and I am not a particularly big guy nor do I have big hands. Remember, this thing was one of the top wedding event cameras for a very long time, so it has some very good ergonomics.
For my purposes I love the Pentax 645Nii and the images I get can truly be stunning. However, it is a much noisier camera than the Canon was using the the motorized lenses. Lens focusing and film advance on the 645Nii are all noisy. But, amazingly, you quickly get to the point where you don't even notice when you are working with them.
I use the 645Nii for landscape, events (birthday parties, kids school plays, etc.) and macros. I do not do sports so I have no idea how it would work for that but it does seem just a bit slow. The image quality is great and I do not have to worry about computers at all (which suits me perfectly.)
Now, the bad. Medium format film is typically expensive, and developing is a lot more expensive, unless you do your own. 35mm is a lot less expensive per frame and you can usually get color negative stuff developed for very little money. As for using the Pentax 645Nii to completely substitute for 35mm...I don't know. I like both for different reasons. Besides, I absolutely love my results from my Pentax PZ-1p and my FA 31 and 77 Limited lenses and am not interested in giving them up.
I've wonder this myself over the the years, and here is my thought process.
1) Standard lens for 35mm format is 43mm, for MF it is 85mm. That is essentially one stop when it comes to hand holding. Moderate telephotos become impractical.
2) Shutter/mirror vibration is going to be greater in MF, further reducing the hand holding speed. (But I shoot OM's, which colors my opinion of acceptable noise and vibration.)
3) The overall kit (lenses, body, etc) is going to be be bigger and heavier. (See note above about OM's.)
The solution? Folders!!
I shoot B&W primarily and color once in a while; I develop my B&W and I'm no Winogrand shooting 3 rolls a day so running costs isn't an issue for me. I plan on using both formats; I just don't know which format I should develop. I have 120/35mm folders in the 50mm focal length but now I want wide-angle lenses. I'll probably pick up an XA or epic because they're cheap and pocketable, but I'm not 100% happy with any of my budget wide-angle fast lens options like the Eletro 35cc and Auto S3 which is why I wanted a 35mm SLR for quality optics and full-manual control. But then I started thinking that I can get 645 SLR instead...
I know the kit is going to be bigger and heavier, but I don't know if it will bother me or not. I figured that If I'm going to be shooting with a camera that isn't compact like a folder/point-and-shoot, then might as well bring home the best negative possible while staying practical (no 4x5!'s)
I have 120 folders and I love them. But unfortunately there aren't any cheap wide-angled options that I'm aware of.
I have Olympus OM for 35mm, Mamiya m645 and an Autocord TLR
It could not replace my 35mm for travel and casual photography but the m645 is a blast to use and you get great portraits with any medium format camera at family events and holidays.
Truly though, 35mm SLRs are so cheap you can have both 35mm and MF.
I've got a Mamiya 645 AFD that I bought to try and replace my 35mm system with and honestly, Like what's been said it's too slow and clunky. The AF sucks, it's heavy as hell, 16 shots/roll (typically), and the lenses are slow in comparison. But good lord the scans look amazing.
I use the Mamiya for more conscious, posed, or slower things. If it moves, if it's snapshots, or it's starting to get dark, out comes the F100.
If you're looking for a 645 with the feel of 35mm, look at a pentax 645 n2. I would suggest also considering some medium format RFs- there are a number of fixed-lens Fuji 645 RFs that perform very well and are comparable in feel to 35mm. Also consider a mamiya 6/6mf/7/7ii, the 6 is my favorite camera overall, no question. It really evokes 35mm in term of handling.
As a former mamiya 645AFD and current mamiya 645 pro owner, I prefer the modularity of the latter. There are many fantastic lenses too. But it annoys me somewhat that the pro is almost impossible to work with in portrait orientation, without the finder and grip. And although it is a very sweet little bundle without the grip and finder, I would not compare it to a 35mm camera. I did treat the 645 AFD a lot like a 35mm system, and did sports and low light stuff with reasonable success. I just didn't care for the AF system and couldn't justify carrying the body around with me when I was almost always focusing manually. The newer AFDs should have much improved AF, if that matters to you.
Anyway, the best thing about the mamiyas is the lens availability- you can't go wrong there. Huge selection- old and new.