A short comparison of the XCR and SCAR can be found here as well... (EDIT: Actually, it was already posted a couple pages back doh)
http://www.ar15.com/forums/topic.html?b=6&f=24&t=295328
Without a doubt, I would take the XCR. Out of the current, available options, the XCR win's hands down. The ultimate assault rifle might be quite a bit more futuristic weapon that doesn't need cartridges, maybe a laser of some kind, but again, for current options, it's hard to beat the XCR.
The SCAR stock is definitely better do to adjustable cheek/comb. HOWEVER, as noted in the linked above comparison of mine, the hinge mechanism blows compared to the XCR.
The SCAR barrel is better because it is a 1:7 cold hammer forged barrel.
As for balance, I don't noticed as much as VB. Mine is a light barrel though, perhaps he had the heavy. On ergonomics, other than the charging handle being able to switch sides, the XCR does EVERYTHING better. The charging handle itself is in a better location, and the thing that really irks me about the SCAR is the horrible AR style bolt release. The XCR nailed the location on this one.
FWIW, I have well over 10,000 rounds of light (range, no classes of any kind, just good old paper targets at my own leisurely pace) use on my XCR, zero problems of any kind. The majority of ammo has been Wolf 55grain. It's my most fired and one of my most favorite weapons.