Upload & Sell: Off
Well, with the 1.4x you can still kind of see the 50mm frame lines. And you'll probably find that the 75 is too close to 50, and will then buy a 90. So the 1.4 will be more useful for that inevitability.
Anyway, I bought the 1.4 to use with 90mm. I hardly use it (the magnifier). It has been discussed in the M8/M9 thread that the proper eyepiece diopter, if you need one, is a more useful purchase than the magnifier. I didn't believe this at first, but now must agree that it also works better for me than the magnifier. I guess it depends on how you shoot with the M9, but if you do fairly frequent lens changes, then going from the 75 to a wider lens will always mean having to unscrew the magnifier, which could cost you time or become tedious (I guess you could use an external viewfinder and keep the magnifier in place for focusing). If you use a diopter, it's necessary to remove it from the eyepiece before attaching the magnifier, then attach it to the magnifier.