What is the difference.  They look the same from the outside.

500 series which is the civilain model, it is slao sold under the Bushnell banner is the basic model. it only come with the hex screw, and it has a build in auto shut down of 8 hours.

510 series is the tactical model which has a protective hood, thumb screw, a bit different electronics and it has a programable auto shut off of 4 or 8 hours depends on the switch you hit when you trn it on.

550 series is the military model, it is the 510 series plus it has the new upgraded rubberized buttons, NV compatible, and it is dive capabile to 33 ft for 24 hours and 66 ft for a undisclosed period.

The "roll cage" on the Eotechs make them more rugged than the bottom line Holosite.  Also, the tactical and military models are available in AA versions.  With the N batteries they have a 70 hr life.  With AA alkalines, 300 hrs.  But with AA lithiums, 500 hrs.

Are they really acceptable for ranges up to 200 yards?  What if one wanted to hit a squirrel sized target?  I don't see how 4x and a 12 or 6 moa reticle would work on such a small target...

let alone 2x mag and a 12 moa, or 6moa... so maybe for less than 100 yards?

EOTech has no magnaification it is what you call in the optic business a 1X.  It has a 65MOA outer circle for quick action shot and a fine 1MOA dot for precision shot.  1MOA bascially equal to 1 inch at 100 yards or two inches at 200 yards, it should be enough to hit any little critter out there.  as long as you can see the darn thing at 200 yards.  in my old age, I can barely see one out pass a hundred.

You shoot squirrels at 200 yards away?  DANG.  In the forest I hunt in I never spot them until they are within 50 yards of me.

For PRACTICAL shooting, the Eotech is quick.  It is more accurate than the iron sights, to me, anyway.  It is easier to center on the target.  This is not a target sight, for that you need a scope with maginfication.  It is a battle sight, and a very fine one.

i use an EOTech 511 on my AR Dissipator.  using it in matches for CQB is awesome...you literally can't miss that 65 MOA circle.
last time at the range i was consistently hitting steel plates at 300 yards with the 1 MOA dot.
really the best of both worlds as far as i'm concerned....