"The Rifleman" had an article recently on the SIG 556. I was sold on the modified Kalishinikov design and accessory-friendly design; however, was turned off after handling a couple. They are quite front-heavy. The trigger is a nice swiss 2-stage job, though, it felt bulkly. Lastly, the flip-up rear sight was this flimsy plastic/rubber thing that just didn't look like something I would fall back on.
I'd love to hear about how they shoot, nevertheless.
The ARs and the M1As are a dream to shoot. I personally have a CCQ-type AR (16" w/ all the other junk) and keep my M1A as a designated long-range weapon.
Eventually you will have them both--so don't worry too much