I like the good old M1907 slings. They are 1 1/4", and consist of 2 loops. You set the front arm loop for the correct size for use as a shooting sling, and then adjust the rear strap for overall length. I set mine for an overall length that is comfortable for shoulder carry, yet where I can still do a "hasty hasty" sling for extra stability (just throwing the sling around your elbow and pulling the elbow out to the side to take up the slack). That's the fastest method for using a shooting sling, but not the most stable. If you have a few seconds to sling up, you just stick your arm through the front loop and wrap your wrist, for a nice steady loop sling that you can use from any position. If you have lots of time, you can put a half turn in the sling when you put it on your arm and cinch down the keepers, but those things aren't really a necessity for field shooting. That's more match style. It is nice to have the capability though, especially if you are setting up on a hunting spot and waiting or something like that. But I always leave the keepers up high toward the front swivel, so the front loop stays open for slinging up quickly.
A good quality leather sling will indeed last a lifetime if you take care of it... but if you are worried about durability, Turner makes a 1907 out of biothane (synthetic leather) called the AWS, which is completely weather-proof.
On my M1A, I have a custom sling setup with the front loop from a 1907 sling and the rear section made of nylon, with the cam buckle from a GI web sling. It is attached using side swivels. I use it for carrying the rifle on my front side in low ready. With the cam buckle I can zip it up tight against my body for retention while running and whatnot, and with the tug of a strap I can release the buckle and snap it back to shooting length. It works great for run and guns, or for any application where you want to be able to retain the rifle secure and tight with both hands free, yet be able to get it into action in a couple seconds.
Here's how it carries:
Andy's Rhodesian Sling is interesting... it looks like it is designed to be used the same way I use my 1907s. Looks like that middle buckle might hold the loop open a bit better than the hardware on a 1907 though. Looks cool. I do like having my keepers available though.
