7 dollars?!?!?!?!?! darn the best deal I've ever gotten with good quality cord i liked was 1 dollar for 350 feet of ok quality jute with dyed in colors.

I prefer natural (or at least natural-looking) materials the most.
My favorite slings thus far are braided from hemp. I also like cotton, but cotton tends to have a bit of memory characteristics, thus why cotton isn't my favorite sling material. (Oddly enough, I find cotton twine far more abrasive than sisal when it comes to braiding.) Sisal is fun to mess with on big slings. You just gotta unkink the sling before use, since sisal is fairly notorious for remembering the twists it was subjected to last. (memory)

Acrylic yarn is the material I've been toying around with lately. It looks and feels like wool, but with added rot resistance. You just gotta make it extra tight and dense when you braid it, so it doesn't feel like trying to sling with a strip of foam.

The only material I'm not very fond of is jute, primarily because of how hairy it is. But who knows, maybe in the future I'll get over that and just run the jute sling over a fire or something to minimize the hairy factor. Hell, I may even love it after that.
*Obviously not trying to knock it, in case he starts to really like using jute in the future *

Hemp or parachute cord is my favorite. I like hemp over jute as it's a bit heavier and a bit more solid feeling.

Update: while it is a bit annoying to braid, as it sometimes want to tangle and stick together, it can be made into a rather good and useful sling.