Not sure what your application is, but the FF3 is taller in the base area and doesn't co-witness with low irons (i.e. centerfire pistols). The RMR and old Leupo DP have less bulk. The DP is discontinued now though, and replaced with the DP Pro. Since this is the Rimfire forum, the co-witness issue probably doesn't apply, but figured I'd throw that out there.
I've had an FF3 for awhile with no complaints so far, but haven't owned it that long.
