Unmounted means the filter is just a piece of glass. You attach it with three small screws (and likely smudge it in the process...).
Mounted means it's in a filter cage, just like almost all the filters you see advertised for visual use. The mounted filter just screws into the EFW.
I bought the 31mm *unmounted* filters with the EFW8. If I were doing it again, I would have bought the 1.25" filters, as the fastest I'm likely to ever get is F5.6 if I use an .8FR with my 115/805.
It's silly to future-proof something like this, or way overthink it. I'd say if you are using fast scopes (F/4 or reduced to that) then get the EFW7 + 36mm filters. If you're using F6 and up, get the EFW8 + 1.25" mounted.
I am buying F5 scope sooner or later, and also i am planning to use my Canon lenses, mostly they have fast aperture, not sure if that is same as in the scopes, but i can go anywhere from F2 up to whatever, but if wide open then from F2 up to F5.6.
And because of what you said, i should be sure i won't go for faster than F6, but i can't see that coming at any time, if not this year then next year, if not next year then definitely in 2019, but if i am planning to spend a lot then getting a scope of F5 is just a matter of time, but let's say if i will force myself to stay with F5 only [or if i will use a reducer to put the scope nearly at F4] should i go with 36mm? Also how much that 31mm may cause issues with F5 or faster scopes?