I've handled the M3s. They seem much heavier than the M1. The whole point of the M3 was to be able to use the pump action for low recoil less lethal rounds, and then switch to the semi mode for buck or slugs.
Neat concept, but to me overly complicated and something I would never use. If you really want an M3, that price isn't terrible. I've seen them sell for a little over $1000 before.
I've picked up used M1s for as little as $575 and recently found another one for $625. That takes some shopping, but you should be able to find one for well under the $900 that the M3s are going for.