Kiah,
I own several Mamiya 7 camera bodies, and most of the lenses for the system, and use them extensively for landscape work. I do own, and have used, the 35mm panoramic adapter, which gives you 24x67mm images on 35mm film. It works quite well, though there are a few quirks, the main one for me being that the dedicated rewind crank has to be fitted to the base of the camera, and indeed screwed into the tripod socket, to rewind each roll of 35mm film. Given that I keep Kirk L brackets on each body, this is pretty inconvenient. I have found that a small coin will allow me to rewind the film without removing the L bracket, so I have a work-around. It is slow, however, esp. compared to the motordrive in the Xpan.
All in all, the panoramic adaptor does work, but it is a little kludgy, and I cannot give it a 100% recommendation. It might suit your style of working, but only you can answer that. It suffices for me when I want panoramics, or if I want to use an emulsion not available in 120 format.
The plus points for the Xpan are:
a) motordrive;
b) ability to take both panoramic and normal images on the film;
c) smaller, and lighter (I assume);
d) availability of a 30mm (M7 lens set only goes as wide as 43mm);
e) Speed of film load/unload
Plus points for Mamiya 7:
a) Longer lens available (150mm);
b) Can also use 120 film.
I really like my M7s, and they get a lot of use. Reasonably robust, but not built to take real abuse. Lens quality is first-rate. Camera bodies and lenses are available 2nd-hand, and sell easily, so expermenting with an M7 system, and selling it on again if you don't like it, won't cost the earth. The same is of course true of the Xpan, so you can explore both alternatives pretty easily.
Good luck with your choice!
P.S. I doubt that 35mm film, or 120 for that matter, is going to become unavailable in the next decade. Emulsions may come and go, but in general, I'm sure, some B&W, colour neg, and colour slide, will remain for us to use. But then, I'm primarily a film guy, and shoot 35mm, 120/220, and 5x4, so I would say that!