I have a Sig556, its a great gun, as long as you throw away the crap factory mags. Trigger is super smooth and I like the fact that I can fold the stock and throw the gun into my backpack. I have probably put 1500-2000 rounds through it and haven't really cleaned it yet (other than cleaning off the factory grease when I first got it), and have had zero malfunctions that weren't mag related.
I also like the safety a lot more than an AR safety (it rotates 45 degrees instead of the AR's 90 degrees). I also like that you can charge the gun in your shooting stance since the handle is on the side like an AK.
The only downside that I can think of is that its not an AR. There are small differences that will make you pause or hesitate when going back and forth between the 556 and an AR since a couple of things are in different places. If that is a concern, the 516 may be for you.
I haven't shot or handled the 516, so I can't speak on it, but if it is as smooth as the 556 and if the gas system is as reliable as the 556, then I would say you can't go wrong with either one.