I've got an E99 and my team mate has a shutter, the main differences after having stripped 'em both are:
- The shutter has a sexy looking bolt, and slightly different cocking grip, the E99 has a straight, plain and simple, functional bolt. Both do exactly the same (interchangeable).
- The shutter has holes in the sides , exposing the bolt, looks nice but lets muck/paint get in. E99 doesn't
- The E99 is Vertical feed, shutter isn't
- Shutter has a shroud over where the barrel screws in, e99 doesnt.
- Shutter has a gauge on the bottomline, E99 doesn't
- Different porting on the barrels (E99: straight, Shutter: spiral)
- E99 has splash anodizing, shutter doesn't.
- E99 has a slightly larger dropfroward that the shutter.
- Oh yeah! E99 HAS AN ELECTRONIC GRIP FRAME! SHUTTER DOESN'T!
The trigger on the E99 can be set to take less effort to fire than it takes to click yer mouse button. And the e99 is cheaper than a shutter. Most if not all of the Shutter bonuses are cosmetic.