i used to use plain split pocket slings, but then made my slings into an apache style by sewing leather or suede over the pocket for a solid pocket. i notice that my range of ammos has greatly increased since then.
i kept my old split pouch slings though, as they are useful for mystifying a competitor. when the watcher can't see how the rock stays in the string held in your hand, it is a little disconcerting. also, it is easier to pass the sling off as a shoelace or some other menial bit of string. imagine this:
slinger walks up to big burly guy that is showing off his throwing range
slinger: hey burly, i bet i can throw a rock further using a string than you can with your bere hands
burly: hah! yeah right. lets see you try
slinger: whips out split pouch thin corded sling and whips a rock really far away
burly:
how did you do that ?!?!?!
slinger: magic. (walks away)
alternate ending:
slinger: i used a sling. here look. (pulls out a bunch of slings and recruits a new member for slinging.org)