Another vote here for moon-clips over speedloaders, loose rounds or other devices. Although reloading is way at the bottom of my list of priorities in a fight as a civilian, I like moon-clips not just for training but for carry also. The case against moon-clips, bent and not allowing the cylinder to close is a "what-if" and I've already stated that I've learned to dismiss "what-ifs" in favor of what actually works best. This principle is relevant beyond reloads. There's nothing wrong with thinking about contingencies and it's foolish not to consider them. I think about them, and then I choose not to compromise for their sake. I can adapt in the moment if I have to, so I'd rather not adapt ahead of time when it costs me to accomodate a "what if" that I'm not assured will happen. In other words "what if not?"
I own 60 moon clips and I've never had one fail. I have bent two or three stepping on them on the ground, but they were easy to repair by flattening again. I carry at least two in my pocket (third one in the revolver) every day for years and they've never been damaged. I acknowledge there are "what ifs" that apply to moon clips.
Moon clips help with ejection as well as insertion, but the way they help is all-or-none. You never get that one case that fails to fall and blocks the incoming. On the other hand, if you get a case stuck hard, they may all fail to eject. You can't pick individuals out. If you cut a head off, the clip will eject but the stuck case won't let another full clip be inserted. You'll have to use the clip like a stripper clip, insert two cartridges and tear the clip off them and repeat so long as you have time.
Speedloaders have what-if's too. Ejecting loose rounds, you can have one stick and need to be cleared before you can recharge. Handling the speedloader, you can dump them before they reach the cylinder. Some of them dump themselves and others just dump because you hit the release at the wrong time. Speedloaders can also fail to release, or they can release and get caught in the cylinder window and take time to clear. I don't think these are good justifications not to carry speedloaders, but someone who carries them should be aware.
Similarly, the what-if's concerning moon-clips aren't fatal. Be aware of possible failure modes and know how to deal with them. Carry what fights best. I want to win and I plan for success. Errors are possible, but I'm not going to give up my advantage to avoid a possible error. I'd rather deal with the error if and only if it happens.