The Sig 556 is heavier. around 8.8 lbs. It is also heavier forward until loaded, than it achieves a wonderful balance. Gas operated with 2 stage positioning, in case of lots and lots of shooting, position 2 gives more venting. USSA (the Moderator) for this thread, is doing a torture test on one currently. He was around 800 rds. with no love or cleaning, and NO problems. Waiting for the next follow up report.
The factory sights, frankly, stink, plan on some type of optics. I slapped a NcStar on it and to 100 yds. it is dead on. Mine also hated Wolf ammo, It may have been too new earlier this Spring, and may shoot them now, but it prefers full power 5.56 rounds, or any other name brand .223. Takes PMAGS, surplus AR mags, and the trigger pull is sweet.
Had a squib round damage the crown of the barrel, and customer service replaced it in 2 weeks for around $75.00 and shipping.
Michael Bane may still have his review on video here in the archives or product reviews. It is good to watch.
Picitinny rails all over it, built very well.
You have a number of good options in front of you, and personal choice and application will lead you to the right one.

Plus it just looks cool.

Let us know how it works out, post pics, and get quick.