I finally got to the range with my ACR, SCAR 16, and SCAR 17.
For me there are only subtle differences between the ACR and the SCAR 16. As noted everywhere, the ACR is heavier. Noticably so when you are running around with. The big plus on the SCAR was how the recoil was more of a push rearward. It seemed shot after shot, it stayed on target where as my ACR would slightly rise and move left.
The couple things I didn't like about the SCAR was that it was sensitive to cartridge OAL. If the cartridge was slightly longer than the 2.260 like 2.263+ it would fail to feed. Where as the ACR would eat anything. The other thing I didn't like about the SCAR was it was magazine "picky". It didn't like Thermold mags, and Pmags, are a tight fit.
In a nutshell if I had to bug out with one of them, it'd be the ACR. It was more reliable and would eat anything in any AR mag. There is no deal breaker between the two...
The one thing, I used the Vortex SPARC on the SCAR. I have no real complaints with it. The one nit is when shooting using the iron sights and looking through the SPARC, it would come on. I guess the shock would activate it. So, I had to keep an eye on it to make sure I turned it off.