Pizza Bob pretty well covered the options, I'll just add a couple comments.
Given how they work, the HKS and 5 Star speed loaders hold the cartridges slightly looser than the Safariland speedloaders, and the 5 Star speedloaders hold them slightly looser than the HKS. One approach isn't better or worse than the other, but it's an area where your preferences or particular use may matter.
The feel is different inserting the cartridges and depending on your technique, you may prefet snugger or looser. Looser has an advantage in terms of fitting different revolvers. For example a 5 Star speedloader for a K/L frame also works just fine on a Ruger Security Six, while HKS has a slightly different sized loader for each (their 10 and their Mk3).
I like the HKS in general as they are available for almost everything and they work well.
However, for concealed carry I prefer the Safariland Comp II as it is slightly more compact and there is no risk of turning the knob while getting it out of a pocket or slip over style carrier and accidentally dumping the rounds. You can guard against that somewhat by training yourself to rotate the unit counter clockwise (against the direction the knob need to turn to open)as you bring it out of the holder but it's not fool proof.
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That said, my preference is to use a moon clip. TK customs will machine your cylinder for moon clips for $175, and the revolver can still be used normally without moon clips. The TK Custom moon clips are very high quality and hold up well in use. Faster than a speed loader and also much more positive on ejection with no chance of getting a rim stuck under the ejector star if you eject in something other than a vertical position.
The gravity issue on loading is less important in my opinion, but it does pay to use a Stress Fire reload procedure to prevent putting yourself in a near wrist lock and in a face down position. You should train to reload by feel while keeping your eyes on the threat and staying in a well balanced position where you can move while reloading.