I prefer the GI web sling for the M1 Garand or M14. Some people like the canvass/cotton sling and some prefer the nylon.
I don't care which it is, both work for me for shooting. I really don't use a sling for carrying a rifle as it's hard to hit the target with the gun hanging off my back.
I've tried the leather slings and, for me, they are more for historical correctness than anything else. I don't use a shooting jacket or gloves and I sweat. That leather sling sticks to a sweaty arm like sweat was gorilla glue. After one range trip with the M1917 I got tired of fighting that leather sling up and down my left arm and took it off. I put a repro Kerr sling on it and it was good to go (you probably don't want a Kerr sling on your rifle, but they look right and work okay on a M1917 bolt action).
I recently bought a magpul two point sling. Big messy thing with lots of extra webbing and the attachment "hardware" is separate/extra cost and I'm still trying to figure out if I like it on my GII or not. It may end up in the drawer and an M1 Garand sling might find it's way onto the GII. I've just got to get the GII into shooting shape again and get back to the range with it.