This was a recent topic of conversation on Weapon Evolution as well. Perhaps the most significant difference is that the SPR/M4 offers the M.I.T.E.R. system, while the M4-2000 does not.
What this means in practical terms is that with the M4-2000 you'll have to zero unsuppressed, and then zero again when the can is mounted, taking note of the shift.
The M.I.T.E.R. system allows you to calibrate the can to the unsuppressed zero, so you get repeatability without the need of making additional sight adjustments. Additionally, the SPR/M4 does a better job of reducing gas blowback through the weapon (and into your face).
Stand your ground; don't fire unless fired upon, but if they mean to have a war, let it begin here. -- Captain John Parker, Lexington, 1775.