I own the Ruger SR556E (w/o chromed barrel and bolt) and it has been flawless after 5,000 rounds. I have never shot the Sig, so I can not give you a comparison there.
What I can tell you is that the Ruger piston system works very, very well. The gas block is adjustable for the type of ammo, which helps keep that spent brass going where it should. It also allows for the use of very light loads and extra power loads. Just turn the gas regulator with a bullet tip and continue fireing without worry! I have yet to have a FTE or FTF caused by the rifle. (I have had a few FTF from ammo).
Many people have cussed the piston guns for bolt carrier tilt and recoil buffer tube ware, but I have found that Ruger has solved the problem. My 556E tube is as pristine as the day I brought it home.
The bolt, carrier, and upper action remain cool to the touch after dumping 3 mags. But beware, don't touch the gas block, it gets scortching hot!
The 556E model does not come with the chromed bolt and barrel, nor does it have the four rail hand guard. The SR556 comes with sites and the 556E does not.
I found that the single rail of the 556E was much easier to hold than the 4 rail 556 model. The 556E does have pre-tapped mounting holes that you can attach rails to in any configuration you desire. The rails can be purchased in three lengths from Ruger. I chose to buy a 3" rail and mount in to the forward bottom of my handguard to mount a bipod on. I also purchased some Troy battle sights to go along with the Lucid red dot I mounted.
I think the rifle was around $1100.00 without the add ons and the sites/rail/reddot/bipod added another $300.00. Not bad for just what I wanted! And the gun will shoot <2 MOA all day, even when the barrel is too hot to touch!
BTW, I also own a few DI guns - DPMS and S&W- and they are fine weapons also.
I hope this helps you out.
Last edited by Wyoredman; November 15, 2012 at 10:54 PM.