I had a SIG 556 Classic but sold it wihtin a = week.
Think of it as a 1600 dollar rifle for features (CHF barrell, 2 positon piston) with 600 dollar workmanship.
The classic had terrible handguards and replacing them w/ a SIG Quadrail would have cost me 200 bucks.
Mechnically on of the advantages of the SIG is it's more tolerant of steel ammo .
Loved the Diopter sight that came with it too.
But all in all it had a real clunky and not well made feel to it, that I sold it and bought another AR instead.
I could really feel the weight difference too, at the time i was weak due to injuries
It is 9 months later and I am much stronger now so maybe the weight difference wouldnt bother me so much now.
If I'll consider another SIG I would only pay the version that comes with the Quadrail already the SWAT.