I've tried both the M2 and the Shelter 901 in my system, mounted on SME V, Michell Orbe SE, van den haul/SME cable, ARC PH3 SE w/ setting at 47k ohm.
M2 was my first cart, recommended by the dealer when I first bought my table. I later purchased a Shelter 901 from Japan on a friend's recommendation before it became popular here in the U.S. The Benz has a higher output than the Shelter.
My impression was that the Shelter 901 had more transparency, better extension at both ends, and a deeper and wider soundstage. It works best with 100 or 200 ohm loading.
At the time, the street price of the M2 was more expensive than the Shelter, so the Shelter was a clear winner. If you can find a 901 for less than $1k, I think it would still be my top choice. But at its current list price, I think there are a few other good carts to consider as well.
FrankC