I am considering getting a SIR for my Preban Bushy M4gery, I cant decide between the #46 and the #50.  I have an aimpoint with a ARMS #22m68 mount. Currently I have it mounted to the flat top with the cantilever spacer.

With the #50 would I use the half spacer if I mounted it on the lower half? Or would I just be better off to get the #46?  Is the bi level rail the only difference between the two?

I say go with the #50! ARMS also sells an add on rail that you can install to make the lowered section the same height as the rails that go over the receiver.  Of course this will cost more money but I feel in the long run you will have more options with the #50.

I don't recall how much the part cost. I believe Ducati960 has one since he posted some very nice pics of this setup not too long ago.  

The difference between the #50 and #46 is that the #46 has a dedicated PEQ2 mount under the foward rail.  The #50 has a bi-level 1913 rail, w/ the ability to level it w/ a rail adapter.  If you don't plan on mounting a PEQ2, you won't get full use out of the #46.  The #50 is more practical to a non PEQ2 user, if you ever want a lower foward rail its there.