Probably the M4 for a couple of reasons. First, it has the quick load system which is much faster than the older system used with the M2. You can get a rapid load system for the M2 and M3, but they're not easy to come by. I also like the faster rewind with the tilted rewind knob and lever, although again you can put a rewind crank on the M2. The M2 has just three frames, the M4 has four - 35, 50, 90, and 135. Neither one has a frame for the 75. If you can, at least handle both.