Recommend doing a search as there have been several very good/long threads on the Eotect vs Aimpoint and of course why not spend the money to get the BAC ACOG.
Have a couple Aimpoints, one ACOG, plus used friends Eotechs on subguns, thus still a newby in this area, however, for what it is worth the short summary (my version ;-) is:
If you are shooting 200 yards and under all the time, with a lot of shooting done fast and at very close ranges (25 yards and under) ... then either the Aimpoint or Eotech work great (friend just got back from the Gulf after using the Aimpoint for a year).
The BAC ACOGs (TA31, TA31f, TA11 or TA11f) work great at a distance (great optics, 3.5 to X4 maginificaition, work at night etc.) PLUS with the BAC aiming (where you shoot with both eyes open) you can still use the BAC ACOGs for tatical applications (be they real world or action matches) from off the end of the barrel to many dozen of yards (at which point the X4 is fine).
The Aimpoint is a little simpler for me, in particular the knob is faster to turn on/adjust and I like the larger 4 MOA dot out to 100 yards. Lastly at this point in time there are a few more mounting options for the Aimpoint to have it co-witness with your iron sights, however, the Eotech is catching up fast.
The Eotech works very well, and is liked by many. The large outer ring (65 MOA) gives many a fast aiming point at close range and the 1 MOA dot is a little finer than the Aimpoints at greater distances than a 100 yards.
While I perfer the Aimpoint, is primarly due to the larger center dot for medium distance (25 to 75 yards) plus having been an Aimpoint user for ~ 20 years.