I cant seem to find out what the major difference is about the A2 or the A3, From the pics at Bushmaster it looks like the A3 carry handle comes off. Do they both have Windage and elevation? Also-how much difference in accuracy does the 16 and the 20 inch barrel differ?

The A3 is a civilian term for a flattop receiver.  Therefore, if the rifle comes with a carry handle, it will be detachable.  Bushmaster's removable carry handles come with standard A2 sights (windage & elevation adjustments).

There is no discernable difference in accuracy between a 16" & 20" barrel.  Both barrels have all the accuracy necessary to outshoot the shooter.  The difference is that the longer sight radius of the 20" barrel makes it a tad easier to shoot it more accurately.  The primary difference you will experience is a slight loss of effective range on the 16 due to the lessened velocity of the round from the shorter barrel.

HTH!

The A3 weghs a lot more than the A2 but if you are goind to use optics its worth it.  Id just skip the carry handle and spend the money on a backup iron sight.