I've got: Harris, Atlas, and PMAG
The Harris are faster to deploy, but less flexible when it comes to adjustments.
Atlas are far more flexible, but slower to get into position and adjust.
The PMAG is "OK" but has a lot of play.
For me it comes down to task and purpose.
Precision work: Atlas.
Hunting: Harris
I'm going to end up buying another Atlas because I'm tired of swapping between rifles.