Whether or not you want to switch, you'll have to decide yourself, all I can do is offer helpful hints
If you don't like the square, and you're cropping to 4:3 or wider anyway, then yes, 645 and 67 make more sense. 645 helps you save on film, 67 gets you bigger potential enlargements. How rich are you vs how often do you print billboards?
Going 645 means you can keep using your current Hassy lenses on adapter (and other brands like Pentacon/Zeiss and Bronica/Nikkor), the only thing in Mamiya-brand you'll need is wider wide-angle lenses.
Going 67 means all new lenses, they're also bigger and heavier.
IQ is always subjective. But in MF there's very rarely any dogs (again, I didn't like my m645 35mm, but I may very well have one that got dropped, so I won't besmirch the whole range because of it).
6x6 I personally find useful, especially with WLF, for street. Square means shoot first, crop to frame later, and no annoying rotating of camera for portraits. M645 is not impossible to hand-hold in portrait orientation, but I've got a busted right shoulder so I try not to do it too often. Best you weigh up your own strength vs how often you currently crop to portrait. And if you don't like square I won't try to convince you otherwise, each to his own.
m645AF are pretty cheap for what they are, maybe $3-500 depending on condition (I think I paid $400 for mine). m645AFD are pretty much the same camera, but they have the ability to add on a digital back, hence they go for more (and the AFDii an onwards are just more expensive again for more digital features).
If you don't want to get a digital back, just get an AF (or even if you're going to go digital 'sometime in the future', getting a digi-compatible body is peanuts compared to the cost of the backs, so buy one then and still just get AF for now).
And to quote the little girl on the Old El Paso taco ads, "¿por qué no las dos?"