You're going to find a lot of guys here that love ACOGs. Personally, I'm kind of blah about them. To me they're a middle ground optic that does nothing well, though they are robust by all accounts. They have too much magnification for CQB and not enough to help you when the distances start stretching out. I can understand the need to fill this niche in some combat operations, but for most of us I think either a high quality variable power scope or a red dot is a better option. I may get flamed for this opinion shortly
. Both of my rifles wear Aimpoints. The truck gun has a PRO with the Aimpoint mount, my "pet" rifle has the M4s with a LaRue mount. Long shots can still be made with a RDS in the unlikely circumstance you find yourself in a long range gunfight. Plus the M4s with the LaRue mount can be had for $800 or so (less with the standard mount) and the PRO (also an excellent optic) for $400 or thereabout. Leupold makes some nice scopes if you're looking for magnification, and some of them like the Mark AR aren't too badly priced. There are many other options as well, ranging from Walmart Centerpoints to $3-4000 German glass and everything in between.
20 inch uppers aren't as common as 16 inch ones, which is likely why you aren't seeing many. 16 and to a lesser extent, 18 inch are all the rage these days. Lots of guys are building SPR/DMR clones with 18 inch barrels.