Back when I was shooting a revolver in PPC competition the vast majority of the competitors were using Safariland Comp II speedloaders, there were a few using the Comp III but I never talked to any of the users to see how they liked them. I don't know about now, but back then I never saw anybody use HK loaders in competition, as previously mentioned the HK's are too slow.
In PPC we had a belt slide that held 3 Safariland speedloaders, and Safariland also offers a spring clip that fits on a belt to hold speedloaders, good for concealment use. I used the belt slide and a couple of the spring clips in case I dropped a speedloading during a match, I would have another at hand and not have to pick it up. Not sure what is IDPA approved.