First off I own both, FDE SCAR and Enhanced CB ACR.
I have approx. 850 rounds through the SCAR (without any issues) I have 0 rounds through the ACR (That will change this weekend). I have shot an ACR, I put a number of rounds through Ryan Smiths demo enhanced ACR a few weeks ago and that sold me on the weapon system.
The ACR wins here, my SCAR has 4 different shades of FDE. The color match by Bushmaster to Magpul's Coyote Brown is excellent.
Again the ACR wins here, parts are already available, the QC barrel is exactly that "quick change". Being able to change out hand-guards, stocks, barrels without tools is a +.
Both guns breakdown very easily, the SCAR may have a slight edge here but losers it when it comes to removing the piston. The ACR allows the piston to be removed from the front of the gas block without the need for the operator to use something as a pushrod to get the piston out as is needed with the SCAR.
The SCAR wins here.
Shooting the Systems
Both guns recoil straight back and are very smooth, the SCAR is very consistent with were it tosses the brass (4 o'clock). The SCAR is much louder and that is a result of the break it comes with. The charging handle is of course different on each of the weapon systems, and while they both have there merits, I have yet to form an opinion on which I prefer.
The SCAR has a 1/7 the ACR 1/9, and to be honest, I could give a rats @$$ about the twist, there I said it.
Cosmetics/overall fit and finnish
The ACR wins here (bigtime) I'm still sporting a scar (no pun intended) from where a SCAR rear sight bit me at SHOT 09. The polymer on the ACR really does inspire confidence, it has a much stronger/sturdy feel that the SCAR. I have no issues snapping the stock closed with the ACR, I will NOT do this with SCAR (I have seen stock locking lugs broken off a SCAR). I replaced the grip and safeties on my SCAR with Magpul products (what can I say I'm a Magpul junky). The storage within the stock and pistol grip on the ACR are nice touches. The ACR also wins on sling attachment points. There has been a lot said about the safety on the ACR, my take is this: I'm right handed and use my thumb to disengage the safety and my index finger knuckle to engage the safety, this requires me to remove my finger from inside the trigger guard before I can do this, it works very well and is safe to boot.
Again I believe the ACR wins here, very easy to use, they are NOT cheep they are inexpensive, that being said I do plan on replacing them with gen2 MBUS when available.
Well I think thats about all I've got,
"It matters not the length nor quality of your life. What matters only is the decision you make for Christ"