The competition is the HK 416 and 417 series of rifles, which is essentially an improved upper receiver for the M-16/M-4/C-7 family of weapons. The gas tube venting gas directly to the bolt carrier is replaced with a short stroke gas piston (similar to the FN-C1 from the last Ice Age, but actually derived from the HK G-36).
I had a chance to handle the SCAR, HK 416 and XM-8 at AUSA last year, and the fact the HK 416 was in essence a C-7 made it immediately familiar and easy to use. The XM-8 was impressive to look at in a movie prop kind of way, and the SCAR reminded me of the C-1, but that really only speaks to me, new soldiers would see it as a new weapon (which it is, the resemblance to the C-1 is very superficial, and fall apart under closer scrutiny).
If I was to choose, I would probably go for the HK 416 solution on a logistics and training basis. People who have fired these weapoons will have their own opinions, of course.