Several points based on my experience, which includes having a clip conversion done to a 66, and shooting that 66 (and a 625) with moonclips in training and competition, and shooting the same 66 with speedloaders in training and competition:
1) Gazpacho is right about extraction; a full moonclip does make this a smoother operation, particularly where you're dealing with a short extractor rod as on a 642.
2) Farscott is right about the PITA of long, skinny rounds like the .38 in moonclips. Actually, it's not so much the long skinnyness that matters (though that's a factor) as it is that the clips themselves are thinner, floppier, and bendier than the clips used for rimless cartridges like the .45 et al.. Unfortunately, this thinness makes any reload(s) that you carry quite a bit more fragile than a speedloader. If not securely held and protected, the moonclip may release rounds, or worse, may bend, meaning that when you shove it in the gun it won't work.
3) Speedloaders (safariland speedloaders, that is) can be made to work very fast with practice (and appropriate grips--with the cut-out left side, and preferably wood, since rubber grabs speedloader bodies).
I know that full-moonclipped .357's like the 8-shot 627 are the cat's pajamas for some competitive revolver games; but IME, moonclips on rimmed cartridges are an idea that's best limited to play guns, which a 642 is decidedly not.
