When I was with AFSOC, I qualified with the SCAR-L and fam-fired the SCAR-H. The SCAR-L was cool to play with but I really didn't see any vast improvement over my M4 and the accuracy was pretty similar as well. The SCAR stayed cleaner in the receiver than the M4 but now you have a piston that needs cleaning that the M4 didn't have. Now the SCAR-H was a different story and I agree with SOCOM's decision to buy more of these rifles. While the M-14 is a great rifle, parts are drying up even in military channels and the SCAR-H definitely fills the void of a rifle with more punch and range than the 5.56mm round offers.
As far as the ACR goes, a buddy of mine had one he loaned to me for a few days to see what I thought of it. Again, accuracy was on par with my M4 and the cleaner receiver and piston that required cleaning wasn't really any different than the SCAR. I did like the bolt release button of the ACR better and have no problems with the 1:9" twist of the Bushmaster rifle as it handles M855 and M193 ammo without any problems. Sure, you run into issues with longer, heavier bullets but for what I stick with, (M855 and M193) it's not an issue. Also, the slower twist doesn't burn out barrels as fast as the faster 1:7" twist of the M4 or SCAR. I did feel that the quick change barrel feature while a neat idea, really wasn't necessary as well as added extra weight for a marginal return. Also, in regards to the barrel, why the M4 style? the stepped barrel is for use with the M203 grenade launcher that as a civilian, you aren't going to have in the first place and while I never tried it, I don't think the M203 is compatible with the ACR even if you did have one. I think Bushmaster could have eliminated the muzzle heavy feel of piston rifles had they gone to a lighter barrel like FN did with the SCAR.
Really I think it boils down to what is you budget and which one do you like better. The M4, SCAR-L and ACR are all pretty much equal in performance based on my experience. Of course, parts is not going to be an issue with the M4 but both the SCAR-L and ACR have far more CDI points going for it. However, if I was buying a new rifle, I think the choice I would go with would still be an M4, mainly based on the parts supply issue.