Magpul MS3. I believe it can be used as a single or 2 point, but if you attach it to a sling plate with a ring that slides left and right, you can shoot left or right shoulder without fighting the sling at all.
I love the MS3. I've also got a Blue Force VCAS with mash hooks on my 308 AR with some attachment points. It works well, but isn't as streamlined as the MS3. I've also got plain jane 2 points on several other rifles.
When I was an AK guy, I stuck to the Suarez International style of weapon/sling interactions. It works well. The key points are good, sturdy, simple two point sling and solid connections. Transitions involve throwing the weapon over your head onto your back with support hand going into the loop the sling makes while drawing your side arm. Easy money.
Now, after transitioning back to the Stoner platform from the Kalashnikov, I've been working on some drills from the Magpul and Panteao camps getting re-familiarized with the ergonomics of a Single point and the M4 in general. I installed and staked the ASAP plate on my Colt, I love it. Ambi transitions are sweet. It rattles though.
We've got AR15A2's and 870's at work, and for range and specialty team training we don't use the slings at all. Most of the shotties don't have slings.
When I first started using the Hasty Sling method during our range quals with the AR15A2, the work rangemaster gave me grief at first.
He's definitely not Jarhead friendly.
My "official" transition with the shotgun when empty is move it to support hand and draw pistol. I'd drop the damn thing if we were actually in the middle of 2 way range action, I don't know how else I'd reload the pistol. I try not to mention reality during mandatory training for my own sake.
For personal use, I envision the use of the sling for 2 point patrol style carry and manipulations for open areas, as well as hasty sling support. For CQB style, single point seems to "feel" better to me. I'm not a big fan of the rifle front sight post whacking me in the sensitive areas though.
Just my $0.02.