1. reliability - Either
2. recoil reduction - Extrema 2 w/ KO
3. durability - Either
4. ease of maintenance - M4, but not much of a difference
I chose the Xtrema 2 w/ KO for myself. However, you forgot the most important factor - personal fit and feel. Mount each one to your shoulder at your local gun shop. Choose which ever fits and feels the best. I don't think you'll be disappointed with either choice.