645 is not much of a change from 35mm and is not worth the effort
Yeah, there was a nice graphic illustration above about why this isn't true (and I know you weren't saying it was, I just didn't want to overquote). Someone said something about "adjacent formats" -- and 35 mm and 645 are
not that.
When there was film for every need, there was a whole roll format in between 120 and "miniature" 35 mm -- "small format" aka 127. The original 127 frame was 4x6(+) cm on 46 mm wide film, later 4x4 square was added and some cameras were made to use half the "full frame" to give 4x3 (which was pretty close to 828, and
still bigger than the 24x36 frame of 35mm). So it used to be that if you wanted bigger film than 35 mm, you could get a 127 TLR (there are some really good ones) and have nearly double the image area, or a full frame (8 on a roll) and get nearly triple, barely smaller than 6x4.5, but in a camera that was literally a "Vest Pocket" fit. These cameras are all over, but the film is not -- try to buy 127 and you get two or three choices, at least one of them hand rolled. If I'm going to put up with that, and before I'll pay the prices, I'll cut down 120 and spool it myself (and get half again as many exposures, up to 24 in a half frame camera).
If you're new to film that doesn't come in a cassette, however, I'd recommend you stick with 120. I've got a Daiichi Zenobia, a scale focus 6x4.5 that fits in a pocket, 6x6 and 6x9 folders as well (one of my favorites is a Voigtlander Rollfilmkamera, from the late 1920s, a 620 TLR (same film, different spool, easily rerolled or in some cameras you can feed from a 120 roll with the spool flanges trimmed). But if it came down to only being able to keep one camera, I'd have to think long and hard between my Speed Graphic (an Anniversary model with conversion Graflok back and Kalart rangefinder, working focal plane shutter, but a little ugly cosmetically) and my RB67. The Speed is lighter, but the RB is quicker and easier to operate, quicker to change lenses (and doesn't require 15-30 minutes to recalibrate the RF, because SLR), and doesn't require a darkroom if I run out of loaded film holders.
For a beginner to medium format, however, something like a Zenobia or Mamiya Six (folder) would be the best choices. Compact enough to fit a coat pocket (even a suit coat), very capable lens, the Mamiya (at least the version I have, there are several) has dual format, 6x6 and 6x4.5 (with captive masks, so they won't have vanished in the sixty-plus years since the camera was made), plus coupled rangefinder, and focuses by moving the film plane rather than the lens or one element of the lens (as many folders do). Keep one folder, that Mamiya Six would win (but I'd be sorry to see the Rollfilmkamera go).