I've posted this in the past, I hope that it will be of some help to you...
My take on speed loaders:
HKS and 5 Star - fine for general range use. Requires two motions to load cartridges - insert/twist. Relies on gravity to chamber the rounds, so the muzzle must be pointing down. BTW - 5-Star release knob rotates the opposite direction from the HKS loaders
Safariland Comp I & II - Better for carry applications where concealability and speed are a concern. Just requires firm insertion to release the cartridges - no separate twist motion. Release button bears against the extractor star to release. Still relies on gravity to chamber the rounds. The Comp I has no top knob and must be grasped by the sides. The Comp II has a top knob making it easier to remove the loader from the carry pouch.
Safariland Comp III and Jet Loader - Competition style speed loaders. Single motion (insertion) to release rounds. Loader is spring-loaded and pushes the rounds into the chamber - does not rely on gravity. Large top knob for easy grasp and removal from pouch (not suitable for CC).
Dade & SpeedBeez (Centerfire) - both of these have a coil spring, around the circumference of the body of the loader, that retains the rounds in the loader. The top knob pushes the cartridge rims past the spring into the chambers. Prone to losing cartridges if dropped - not real positive retention.
Variant SL - I've never seen these used, but from what I've heard these were excellent. Unfortunately, it appears that the lone importer for these is no longer bringing them into the country.
The loader you choose may be dependent on application (comp v. carry v. range) or availability of a particular loader for a particular cartridge/firearm combination, so optimum applications may not always be achievable.
As an alternative, companies like TKCustom can machine the revolver cylinder to accept moon clips without losing the ability to still use speed loaders or load rounds singly.
Adios,
Pizza Bob