The price difference between the T1 and M4S comes to about $100 for the M4S at $727 and the T1 for $625 -- prices include mounts:
http://stores.homestead.com/Laruetac...ail.bok?no=149
http://stores.homestead.com/Laruetac...ail.bok?no=226
Both RDSs are NVG compatible, and most of us don't need that capability.
The H1 is essentially a T1 without NVG compatbility. Have you thought about the H1 with a LaRue tall mount for $542 shipped?
http://www.smartgunner.com/MicroH1.aspx
LaRue offers discounts to LE, just as Grant does does for M4C members. With the discounts in play, there may be significant savings.
The H1 is an excellent RDS and suits the needs of the vast majority of Aimpoint Micro users. Housings and construction for the T1 and H1 are identical, so don't let the submersion specifications fool you.