Owning and shooting an SL8-6 and a number of AR's I think that it is difficult to compare the two as they are very dififerent. They share a caliber, but beyond that I see little similarity. I will say that my SL8-6 has been 100% reliable (I can say the same about my AR's) and is extremely accurate. Given the choice between a "stock" AR and an SL8 to do longer range shooting I'll take the SL8 every time. For combat the edge goes by far to the AR. The SL8 isn't that kind of weapon. The SL8 has potential to approach a sniper type weapon, but beyond that the AR rules.
One afternoon I shot my SL8 with an Army sniper and he was extremely impressed with it noting that with a bit of trigger work and the right scope it had great potential.
Many will point at the stock and capacity of the SL8 and criticize, but I find the thumbhole stock to offer a very positive grip, almost like I lock in. Some can't see beyond the politics of the thumbhole stock to recognize that what HK has done may actually be very effective, but, ergonomics are very subjective. As to capacity, the factory mags are limited to ten rounds, but ProMag (yes, ProMag) makes an extremely high quality 20 round mag that functions perfectly. Of course, they are single stack, so the gun becomes more cumbersome and problematical when shooting from a rest or prone.
And, that leads to the final point. An AR can be a tactical rifle used anywhere, and handily, from the desert to clearing rooms in an urban environment. The SL8 is not that kind of a rifle and would be ill-served for that type of a role. If they had chambered the SL8 for .308 you might have the platform for a sniper rifle or an even better target type rifle.
The SL8 and AR15 are apples and oranges and any comparison is really of limited value as the guns are generally for very different uses, but, I guess that is a valid comparison in and of itself. The AR15 is far, far more versatile.
SL8-6 with 20 round mag: