My experience is with the Swiss PE90 series of rifles, but I've owned at least one variant of all 3 rifles you listed, so I'll chime in anyways.
The trigger on the Swiss PE90 is IMO better than most AR triggers, not as good as a match AR trigger, but it is nice for a stock trigger. The trigger on the SL8 is also very nice for a stock trigger, and I find that most people who shoot the PE90 and SL8 side by side prefer the SL8 trigger.
Reliability goes to the Sig. It has a long stroke piston and 2 lug rotating bolt. Very AK-ish. It also has a 2 position adjustable gas system (regular and high, for when the rifle is highly fouled or weather conditions are bad). This isn't to say that the AR-15 and SL8 aren't reliable. Their track records have proven otherwise, but the edge goes to the Sig.
Accuracy is acceptable, you should be able to get close to MOA, may be slightly under, with match ammo or quality reloads. That said, the SL8 can match this, maybe even beat it, and a quality AR can definitely beat it. The advantage the SL8 and AR have is the barrels can be free floated, while the Sig cannot.
Weight is the last thing to consider. The Sig 556 DMR (according to the Sig Sauer website) is heavier than a comparable AR and the SL8 (which weighs 8.6lbs, according to Wikipedia) by about 1lb.
None of these rifles are really "sniper" rifles, but they can all serve the role of a DMR (engaging a man sized target out to at least 600 yards).
