Practical use? The web is probably better. During WWII, the slings on Garands were used to carry the rifle. The use of the sling for support as we do in highpower shooting wasn't really done during combat. It's possible soldiers adopted a 'hasty' sling support position, and if so, the web is the best sling for this.
Highpower use, it's all personal preference. I tried the web sling, but I prefer the leather sling. Frankly, I find the leather sling easier to adjust and easier to find the sweet spot for sling tension.
Here's instructions on how to use the web sling:
http://www.ray-vin.com/tech/websling/webslinghelp.shtml
Here's instructions on how to use the leather sling:
http://www.ray-vin.com/tech/slinghelp/slinghelp.htm
Note the leather sling method above is not the original/standard method designed by the Army. It's a modified method that many highpower shooters use. Jim Owens of
http://www.jarheadtop.com outlines this method in his book "Leather Slings and Shooting Positions". I use this method.
The web sling and leather sling should be 1-1/4" in width (someone correct me if I'm wrong about the web sling width). Length wise, I think webslings pretty much come in the same length if you buy original USGI slings. Leather slings on the other hand come in varying lengths. I personally use a a 50" Turner sling, but I'm shorter than most people. Lots of people say that you'll need a 54" if you're above average height.
Note that when you get a websling, you want a USGI sling made out of cotton. If my information is correct, webslings were manufactured in the 70s and after that were aftermarket web slings made out of some other non-cotton material combinations (ie: nylon). From what I understand, these are garbage since the buckle doesn't grip well on the sling when using them for support during shooting.
The baseline standard of leather 1907 / National Match slings are the
Turner Slings. There's a guy named Les Tam that everyone claims makes the best leather slings. He doesn't have a website, but if you google his name, you should be able to find contact information.