I know you asked for an opinion between the S&W and the Ruger but I've always thought the Colt 6920 OEM carbine was a pretty good deal at $650.

I wasn't a huge fan of the Colt rifles when they went to all of the non-standard parts back in the 80's but the new ones are about as mil-spec as you will find. I also kind of think it's just sort of cool that the lower is marked "M4 Carbine".

Colt LE6920 OEM AR-15 Rifle 5.56mm 16in Black No Furniture

Yes, you'll have to get the furniture you want for the one in the link but chances are you'll end up swapping out the furniture on one of the other ones anyways.

Not trying to start an argument and I am sort of an AR snob, but buying a S&W or Ruger AR-15 is kind of like buying a Taurus or a Rossi revolver instead of a S&W. Sure they're cheaper and they look a lot like their more expensive counterparts but it just isn't the same.