Thoughts
I have both a full Mamiya 7 kit with all the lenses, and a GSW690III (had a GW670III too for a while). The only thing the GSW adds is 12mm of film width, over the Mamiya 7. The M7 is better in all regards, really. The GSW lens is fantastic, no doubt. Sharp to the corners, no problem with film flatness in my experience, and really just a great rugged camera body too. But the Mamiya 7 50mm is the sharpest lens in that system in many regards, has better contrast, even sharper into the corners, and you already own the Mamiya 7 so it makes more sense to get the lens. If you really want to up the ante on the wide end, skip the 50 and go for the 43mm. It's even better, if it can be believed. It's really *the* lens for that system, and in many ways, the 43mm is the reason to own a Mamiya 7.
I think the APO componon 90mm would cover the GSW negative, btw. It's only 58mm x 82mm, and that should be doable by that enlarger lens.
Still, bottom line, I'd say get the Mamiya lens.
-Ed