Have struggled with the same thoughts over the last year or so, and have been lucky to go out with someone that has used both 7x17 and 12x20 (scootermm). So here are my impressions from watching him work (and he makes it look easy). The 7x17 is a pano format, it feels wide, yet Matt managed to shoot both horizontal and vertical. With the 12x20 (very close to the golden mean BTW) the format does not feel as wide. Both 'look' good when printed.
You mentioned that you are also using some of the alt process, does this include plt/pld, if so consider the amount of metal required to coat a sheet of paper, size of contact printer - I would not recommend a split back contact frame for 12x20, a vacuum frame for sure. Trays for the 'big' prints, negative sleeves, etc all need to scale up.
Not trying to discourage you at all, more of what to expect with the step up - you may need a new darkcloth, bag to carry it in as well as new lens and tripod. A Fuji 450C or Nikkor 450 and a 355 (g-claron maybe) would work great for 12x20 or 7x17. In the end, I opted for 7x11 - because I like a slightly smaller format - it works for me, but I do like the perspective of the 7x11. Monty's thoughts about 10x12 are very good...it would actually give you a 5x12, 7x11, 8x12 and 10x12 or just about anything else smaller just by cutting the film down or having a back made for the camera.
The Ritter or Chamonix would be an excellent purchase. Noted that you are in Japan, so the issue of what to do if you want something modified, or if something breaks is not the same as it would be here in the US. Richard sales a DVD instructional video that details how to take one of his cameras from horizontal to vertical, it's pretty cool how he designed the cameras. Changing formats is pretty simple too, like a Canham you just change out the back and bellows. If it were me and I could afford the camera, film, filmholders and had the room, and the back to haul it all...I would go with the 12x20 - then you would have a large number of formats to work with. With 7x17 you have 7x17 and 7x11 and say 3-5x7's per sheet.
Good luck, and have fun..