I've used both extensively. They're both excellent and there is no way you will be dissapointed in either, so it comes down to personal preference and rifle configuration. The EOTech is a bit more power-hungry and slightly heavier, but it has the best reticle in the business, IMO. The ACET Aimpoint ConpC3/CompM3 series are lighter and they can live on the same 1/3N power cell for years. The CompM4 is more rugged and its AA power lasts even longer than the already insane long-life you get from the others, but its heavier.
The CompM4 comes with everything you need to mount it on the AR flat top, as do the Eotechs. The other Aimpoints need a QRP ring with spacer or the ARMS#22M68 with spacer etc, to get them high enough for the AR flat top.
If you're mounting on a carry handle, the formula changes in favor of the lowest-mounting sight, which will be the Aimpoint CompC3/CompM3 series sights. You can put any of them including EOTech on a "gooseneck" mount, which puts the optic forward of the receiver and much lower, which is good, but they tend to be slightly less stable (or that is what we're seeing here). If you have to use a gooseneck (forward carry handle mount) we've had the best luck with the Aimpoint 10174 and QRP (11884) combination. (Sorry EOTEch, I love you, but that third-party, fixed forward carry handle mount is junk. All other EOTech offerings have been superb.)
Mounting Aimpoint CompM3 et al on top of the carry handle works well with the ARMS#16 alone, or the ARMS#2 with Aimpoint ring. The EOTEch will need a rail like the ARMS#2 and will sit significantly higher.
That's my personal experience. Others may differ.
