Unless you're going to be carrying your gun all day, don't get a sling. Save your money for training and practice. For 99% of AR owners, the sling only serves to get caught on other guns as you remove it from the safe or to get tangled on door knobs and other protuberances.
However, if you just have to have one, there are a couple things to take into consideration. What is your main purpose for the sling? How will it be used?
If you're just going to use it to carry the gun from the car to the bench at the range, any web sling will be fine. I recommend this one:
Universal Tactical 1.25" Sling - BLACKHAWK! It can be found for around $7. I use one of these on my shotgun at classes that require a sling to carry the gun. It comes off when I'm back home.
If you have a more practical purpose, like 3 gun competitions or advanced defensive purposes, a quickly adjustable sling is necessary. Both the
Magpul Gen 2 slings and the
Savvy Sniper slings fit this category. The Savvy Sniper slings are very well thought out, but cost about twice as much as the Magpul. I have the Magpul MS1 and it has been a very good sling.
The quick adjustment means the difference between pain and comfort when transitioning from one gun to another or when moving. Being able to tighten up the sling when you're not using the rifle, but moving, means it will keep the rifle tight against you and keep it from banging into body parts as you move or run. It also helps adjust the sling to be used as support for longer shots.