From Magpul on ARF:
-The M3 design is stronger in all tests over the current PMag (feedlip impact, spine strength, side impact, cold/hot weather. etc.).
-The M3 has geometry changes to increase reliability in extreme and adverse conditions.
-The M3 has new low drag follower design for increased reliability in dust environments.
-The M3 is now drop free compatible with magazine wells on the tight end of the tolerance.
-The M3 is now fully compatible with HK416, SA80A2 (Magwell length) and FNSCAR (Bolt Catch) weapon systems.
-New over travel stop allows the M3 mag to be used in mixed HK416/M4 environments without potential over travel in the M4 weapon. (previous PMag has the over travel stop in the front of the mag which cannot be used due to the longer HK416 magwell).
-The M3 magazine catch area is reinforced.
-The M3 magazine has a new slimmer floorplate to aid in pouch compatibility.
-The M3 has slightly increased rib height for better handling under stress.
-The M3 has grip ribs on front and rear spines for increased positive handling under stress.
-The M3 has a recessed paint pen matrix for individual marking of magazines that is consistent and will not become illegible due to use and cleaning.
-The M3 Maglevel features new window material which is more chemical resistant and has a new non glare finish.
Not really a slickdeal, these are just pre-panic, read:non-sucker prices. Not what a slickdealer should pay.
The timing is also questionable, was AIM going to sell these for double if the bill passed? No idea, but odd coincidence.