The ultimate in robustness and reliability is the F. Why not use it? In my view you'd be crazy to use anything inferior, such as the FM2, which is a jumped-up Nikkormat
I've just checked both my black Fs and there's a foam mirror stop but it's in perfect condition on both of them, i.e. it hasn't dissolved into nasty crumbly dust. Light sealing is done properly, i.e. precision engineered labyrinthine: no foam. Or if there is, I've never noticed it, and I got my first F (which I still have) second hand around 40 years ago.
Cheers,
R