We used the M1.5s on the 5973 units we have but all died far too soon. I even placed them on the floor instead of tucked up under the MS and it didn't help with the life of the pumps. We had several M2s left over from the 5995, 5970, 5971 and 5972s that were replaced by the 5973s so I just reused those. None of the M2s have ever been refurbed and a couple have been here since before I started working here in 1991 and are still going strong.
The db of the M2 may be higher, but the frequency of the sound is lower so it is not as noticeable as the M1.5. They also run cooler which is why I think they backstream less and live longer. One of the 5975s we have came with the small Pfeifer pump and it would fill the line with oil half way to the turbo pump even with the rough pump sitting on the floor below the MS. That now has an M2 on it also. One note, put a regular plug on the power cord and plug it directly into the wall, because the motor on the M2 does pull more amps than the M1.5 and best to not overload the breakers in the MS. You have to unplug them when you want to vent, but otherwise everything else works the same.