Don't get me wrong both the SCAR and the ACR have a high cool factor, and if someone offered them to me I (or even sold 'em for around $1500ish and I had money handy) I wouldn't turn 'em down.
Those guns just seem like half hearted attempts to replace the M-16/M-4 family in the military, IMO the M-16/4 family will be around for a while longer. My guess for a real upgrade is some kind of bull pup, probably caseless ammo too.
...That or some nifty gauss gun thingie like in Mass Effect. Probably the bullpup though.
"This is not about land or money...but the one thing that no man should never be able to take from another man: the freedom to make his own choices about his life, where he'll live, how he'll live, how he'll raise his family. "
William Travis, The Alamo: 13 Days to Glory