A piece of advice I would add would be for those who use a sling with someone's name attached to it -the BFG/Vickers, the VTAC, and the Proctor Sling come to-mind immediately as great examples, but remember that there are also excellent slings which have very specific design parameters that are built/designed by those who don't put their names directly on the product, yet it still has amazing pedigree, like the SIERRATAC- to look up any instructions/tips that the "famous name" person or designer/maker may give regarding the sling that bears his/her name.
Similarly, if you have the opportunity to watch them "run" that sling - or even better, when you actually take a lesson from them - pay attention to exactly how they set up the sling when static and also how they use that sling in a dynamic manner. Many times, these individuals incorporate design parameters/features that specifically work towards their personal preferences, and these nuances may not be immediately apparent otherwise.