As a guy who gets to the range 1-2 times a month with both guns (my buddy owns the SCAR), I have pretty regular exposure to both. We shoot them both back to back each time.
The SCAR has the best feel/ balance of any high capacity rifle I've picked up. It feels much lighter than its spec sheet would have you guess. The recoil and muzzle hop are almost nonexistent. It seems to be a very reliable rifle as it should be given the extensive T&E it has gone through.
After shooting the SCAR and then putting rounds through a 556 SBR with a Levang flash hider, I found myself wanting the SCAR's flat and low recoil shooting in the Sig. Swapping to the SCAR's muzzle device, the PWS FSC-556, gives you much of what it offers in that regard.
I brought out a new green full length Sig 556 classic (with the PWS FSC-556 on the muzzle) and the first thing that the SCAR's owner said was that it had less recoil than his SCAR. So you can get much of the excellent shooting feel of the SCAR from a 556 but you won't ever get a 556 to feel as light and handy overall.
I'd guess that the SCAR has an edge in accuracy if you shoot for maximum precision at longer ranges. In a 'minute of bad guy' fire fight situation the difference in accuracy wouldn't be noticeable.
Both guns offer the same reliable shooting performance mag after mag. One does it in a less versatile, less handy, slightly less ergonomic and much less expensive package. In fact, you can pick up a classic 556 and get a free 522 with it for increased training time. There is a lot to be said for that.
Looks being subjective, that is still another major difference. The Sig is very conventional looking and the SCAR falls into the latest chunky wondergun profile which seems more love or hate. I personally prefer the chunky looks of the SCAR but looks don't carry much weight for me when I make a decision on a tool like a rifle.
From the latest range trip...