I want to a point I didn't see mentioned about the 67 cameras - the negative aspects of a giant negative
The epson flatbed scanners use a strip film holder, so they scan 645, 6x6, 67, 6x9 with no difference. My 16bit b&w tiffs are 280mb from a 2400dpi 645 scan, so you are looking at 500mb digital negatives for the 67. If you have a modern computer with plenty of ram (8gb or more) you'll be okay, but I wouldn't try to edit one of those one a machine with only 2gb of ram. Now, you can drop the dpi to 1200 and cut that by 4 and still have good looking images; they will look fine on facebook, but your prints won't be nearly as nice as they could be.
If you don't already, at some point you will want to see your MF images enlarged on traditional photopaper. I find making a good looking print is much easier in either a fully digital or fully analog process. You can still get a good print from a hybrid process, but I find (even as a complete noob) it is far easier to get an amazing looking print from 645 in the dark room than scanned, edited, and sent to the local lab. Getting a good looking 8x10 print from 645 is much easier than from 35mm, so you'd expect me to say go up to a 67 for something even better - but that only works if you have access to an enlarger that can handle 67 negatives, and many can't.
You'll find that at the amateur level (ie not absolutely huge and heavy), most enlargers can only take up to a 6x6 negative. Since I'm in the process of setting up my own darkroom I'm very grateful that I went with 645 and not 67 (like you, I had considered a 67 system at one point). Since you need to crop a 6x6 for an 8x10 or 11x14 print, there isn't much resolution difference between 6x6 and 645 (unless you are printing square). If you print a 67 negative on a 6x6 enlarger, you'll be cropping it down to 6x6 in the negative carrier, then down to (almost) 645 when you put it on paper. So why bother with the extra cost and weight of the 67?
Now, if you have access to an enlarger that can handle the larger negative, by all means, go for it. If like me, your darkroom dreams are realised in a small photoclub darkroom or a bathroom with a portable enlarger, 645 is a better choice.