The MARS is a battery powered sight with IR laser. THe similar optic is the Insight ISM (the XM8 sight) - THe insight ISM retailed at 1000USD while the M21 is a 280USD item(and no one is buying). The one MARS available in the US got sold at 1400USD -Personally I think the price is retarded.
If you toss the M21 into garbage where it belongs to, and buy an Aimpoint and a mount for it, you will be out 800 bucks......there is very little market for the M21, since you will have to buy a picantinny mount for the M21 to mount it anywhere else(the mount for TAVOR is a bit too high for aR)
I would bite the bullet and get the MARS- since there is 3x pocket scope and NV designed to get on the rear mount on the butt. If I go with Aimpoint or Eotech 557, it needs a long rail for the 3X scope add on, and the rear sight is in the way. Unless you want to hack the rear sight.
Either way, if you don't like the M21, you will be out at least 800 dollars to fix the problem. So adding 200 bucks to get the MARS is in reality not significant to the overall cost.
This is one expensive rifle - like the Swiss arms, you are not going to be done without 4K to get the configuration you want. It is basicly designed around the MARS. At least if you don't like the MARS, the market price is 1400 USD while the M21 is worth nothing at all. If you decide to toss the MARS, your rifle ends up costing 2600 bucks while if you are stuck with the POS M21, you rifle is still 3000 bucks since you cannot recover anything from the M21