For positional shooting I would recommend the TAB Pinnacle rifle sling (PRS). What really made the difference for me was the video Tony made demonstrating the use of the sling. Without an arm loop, Tony's technique is simple and allows for tremendous flexibility in moving from position to position. The design (and this isn't unique to the PRS) was intended for the shooter to get on target, then with the off hand tighten the sling. This gets you about 90% of the tension you need, the remaining you lock down with your elbow, or rotate your shoulders to lock in.
With some practice, it becomes second nature and I find myself using the sling in nearly every position except prone. There is a webbing loop if you want to attach the rear of the sling to a belt carabiner for shooting off a barricade or tripod. If your intended use is offset the weight of the rifle in a patrol carry position for extended periods, you may want a bungee style sling.