Assuming you are making the most of 35mm, medium format will give you slightly more sharpness, and a little more detail, depending on the print size of course. Tonality - in my opinion the differences are trivial. The real step up in that area is to sheet film. And of course, technique, since tonality has a lot to do with printing skill. John Sexton's prints from his Hasselblad negatives look great. You might mistake them for large format. On the other hand most of the prints I've ever seen by other people using medium format look like they might as well have been made from 35mm negatives.
So I guess you can put me in the camp that doesn't see a whole lot of benefit to medium format over 35mm. I never much cared for the square negative, so I ended up doing a lot of cropping anyway. And the other types of medium format systems out there are often quite limiting in lens selection. Further, I didn't find medium format all that much less clunky than my large format gear. So I went back to 35mm when I need the convenience and use sheet film otherwise.
I do notice a real difference in tonality between 35mm and 645. I somtimes show my portfolio on my iPhone, and even at those ridiculously small sizes, friends comment on the tonality in shots taken with my Bronica ETR (as opposed to the 35mm and digital work in my portfolio.)
Between your two choices, I suggest the Mamiya as a primary camera. Having owned and used a number of folders (AGFA Isolette, Zeiss Super Ikonta, Moskva5) I can't recommend any of them as your only camera. They are fun and can be quite handy (they fold after all!) I do get the occassional really nice shot from the folders, but I can't count on them to deliver the goods consistently. Things that conspire against getting a high rate of keepers include:
* a folding mechanism that means the lens and film plane aren't aligned to the same tolerances as a rigid camera
* often older, uncoated lenses
* focus issues exacerbated by a lack of rangefinder, or a rangefinder that is small or seperated from the main viewfinder
* often a small viewfinder window that makes composing more difficult
* shutter release ergonomics that invite camera shake
* big springs on faster shutters can induce shake in the lens/shutter unit (Moskva 5 :-)
My Super Ikonta fares the best of the folders I have, but I still only get about a 75% keeper rate (from a technical point of view.) My Bronica ETR however, will deliver 100% of the frames in focus with none of the issues that crop up on the folders. So important stuff gets shot on the Bronica, and I keep the folders for more casual fooling around. If a system camera like the Mamiya seems like too much bother, then I would suggest a TLR instead of a folder. A nice clean Yashica has none of the drawbacks of the folder and can be a real pleasure to shoot with. The results from my own Yashica LM have bested my Bronica and my Rollei 2.8F.
With regards to the difference between 6x6 and 645, it is a bit of a moot point. Especially if you print to the common rectangular sizes, you will be cropping your 6x6 negs to 645 anyway. The biggest difference between 6x6 and 645 is that the 6x6 camera doesn't need to be turned on it's side to change the orientation.
Certo6 appears to do a good job fixing up folders. I think a TLR is more practical though, and I have two TLRs (yashica and rollei, not that it really matters, and neither of which is worth more than $250) I have used but not owned medium format SLRs, so listen to others regarding that; pentax 67 series is a respectable option too.
Good tones come first through good film developing and film. Consistency in mixing, temperature, and agitation so as not to over/under cook things. I really like 12 shots per medium format rolls. I'd scoff at 12 shots per 35mm roll though. I mostly use tmax400 in pmk or pyrocat hd for film/developer. If you change films all the time you won't figure out good tones.
In the darkroom, if you don't have a el-nikkor/rodenstock/scheider componon-s enlarger lens, you might spend the $50-100 to upgrade. Final results of the tones depend on the output paper as well. A good paper won't make a bad image sing, but it can make the great images even better. Tastes differ, but I'm liking alternatives to normal B&W vc rc such as ilford mg rc warmtone, ilford art300, fomatone mg classic.
If you're on a budget, find a good, used Rolleiflex, Rolleicord or Yashica and spend a little to have it serviced if needed. You'll have invested a few hundred bucks in a portable camera that will tell you if you want to pursue MF for the long haul. You will find the negatives give you the resolution, detail and tonality that is much better than 35mm. Give the format a good trial and take it from there. You may find you want a system that is more flexible and expandable, or you may find you want to stay with 35mm. Either way, you've not invested a whole lot of money. Save the cash for film, paper, chemicals, enlarging equipment.
Put me down as another TLR fan. They're much more handholdable than most non-folder MF cameras, reasonably portable, and just plain *cool*. I also really like the square format (which seems to be a love-it-or-hate-it proposition for most people).
I also agree with the pro/con list---you just kind of have to weigh those criteria against one another, and decide if the balance justifies the expense (which for a good folder or a respectable TLR can be pretty modest).
I'm with Michael R; skipping over MF and using just 35mm and LF works well enough. A 4x5 negative can be scanned and printed if one doesn't want to invest in a 4x5 enlarger. Edward Weston sold 4x5 contact prints. The lightest 4x5 cameras are little larger than some MF gear. Film holders do quickly add to the kit, though. We tend to work slower and take fewer shots with the larger film.
I have seen a huge, something like 30x50", print made from Ilford 3200. It was amazing. Yes, there is an advantage to MF. I have enlarged a 645 shot to about 36" wide, and it looked really good. Lots of lovely detail, yadda yadda.
Every camera has trade-offs. The 6x6 format is good, and Yashicas are good and light. I have a Yashica 635, and it produces good results. I have a Fuji 645, a Pentax 6x7, and a Pentax 645. All good cameras, each with something special. If you want a folder, you may want to consider the Fuji folders. These are excellent cameras. If you want an older folder, what Mr. Kreckel sells looks good.
You might also consider (gasp!) a Holga. You can use it, have a bit of fun, and you only spent a little bit of money on it. They can produce reasonably decent images, and you will never care if it gets dinged up. The only drawback to it is when you have to make a lot of snaps with it in a short while. I used one to cover the Occupy Seattle's first big event, and the film advance knob left my thumb and fingers red from the effort. But otherwise I take it when I know generally the light conditions, and the camera could get banged up easily.
My recommendation: set a small budget and stick to it. There's lots of fine choices for cheap.
Matt- Go for it! I'd recommend finding a cheap 6x6, either folder or TLR. You can often find an Agfa/Ansco for around $25US. A TLR can be had for more, but the important thing is to get started. If you like the results, you can upgrade as funds become available. Until then, you can hone your processing/printing skills. The limited number of exposures per roll will help sharpen your compositional skills.
Then, in 2013, you can ask about 4x5. And, in 2014.... :-)