Only you can decide what you want in negative size. I am not a big fan of 645 since it is not that big. Granted, it is bigger than 35mm. I am not a big fan of 6x6 because as already stated, it ends up most people crop to more like 645 for printing. I think it was ShadowFox that mentioned you get to pick what part of the negatvie you will crop from, and that is very true. I used to do that often with the Yashica MAT 124 G I had many years ago. I really loved that camera because it was the first serious Mf I every had and I loved the idea it had a built in light meter that was quite usable.
However, having gotten a Mamiya Super Press 23, a 6x7, I really like it. (of course no one on this forum would know that. 😀 😀 ) What I like about it besides the fact it is a 6x7, is that it is a system camera. It has interchangable lenses and backs, perspective control with a bellows back, extension tubes for closeups, framelines for 100mm (the standard), 150mm and 250mm lenses, and that with changing frame lines for closeups. The other lenses require an auxillary viewfinder, but are rangefinder coupled (the one exception is the older 250mm f/8 which is not rangefinder coupled). All lenses are tack sharp. The 50mm is equivalent ot 24mm on 35mm cameras, the 65mm to 28mm, the 100mm is normal, the 150mm is like a 75mm, and the 250mm is like a 135mm telephoto. All lenses have their own leaf shutters, from B to 1/500, so flash is to 1/500 synched. All also have a method of keeping them open for focusing on ground glass. Since all lenses are leaf shutter, they will be quieter than an SLR.
You can get 6x7 and 6x9 backs, but the 6x9 are more rare unless in the triple format of 645, 6x6 and 6x9. All backs take 120 or 220 film. The 6x7 backs are more common, and wind on with a crank like a 35mm camera.
The camera should come with a handle mounted on the left of the camera. There is a button in the handle that controls a shutter release cable for all lenses. The backs extend far enough to grip with the right had for gripping stability. I have found it quite 35mm like in use, and very hand holdable.
Being a 6x7, it is a little heavy, but not as much as you would think, and a wide strap helps that a lot. It isn't easy to get cut film, but there are 3 different focusing backs, two of which take cut film holders. Sorry, no built in meter so you will need a separate light meter. I used a Sekonic L28c2 for years, and the Gossen Luna Pro is also good.
Well, I suppose you think I like the Super Press 23. You are right. Of course there is also a Universal model which lacks the bellows back but with an adapter will take gralock cut film holders or roll film backs. What's not to like?
But, maybe not for you. Like all other MF systems, they are coming down in costs. You can find the newer and older models on fleabay all the time. I don't recommend the olders one for both their age and esthetics. Due to their age you may end up needing cla, but that is true of all older MF systems you may choose. I have had good luck with mine. I have had mine with the 100mm f/3/5 lens for over 30 years with no real problems. I purchased the 65mm lens, then the 50mm, then the 150. Only the 65mm gave me a problem, with the shutter jamming. The others have been fine, and the 50mm I know was used commercially before I got it.
Good luck in your choices. Frankly, I don't think you willl hate any MF system just because of the negative size. The problem comes if you keep wanting to get bigger negatives is you start with a 645.
