I have a stable full of the little Colts myself and a couple of Smith J frames. So far I've managed to stay away from the K frame snubs (except for a model 66 I owned for awhile), here's my take on where they fit in. And remember it's only my opinion.
For pure carry in your pocket unobtrusively and just about any sort of deep concealment the J frame in king! They're light, potent and very accurate, which is why I prefer my model 60 for the above mode of carry. The downside; they're a booger to shoot! If you put on the bigger grips you lose concealment, so my 60 wears Smith wood plus a Tyler T-grip and that's it.
The D frame Colts are the next up in size and I love the way they shoot! Plus most of the Colts are +P rated, and they hide in bigger pockets rather well. Problems; First as most have said they're getting RARE! I have a parkerized Agent that gets used but my other horses mostly stay in the safe. Second they're actions are almost as smooth as a K frame, with the exception of the slight stacking just before let off. Third it's a bear trying to find a gunsmith that can retime a Colt!
The mighty K frame; Truthfully I'm still scratching my head trying to figure out where these guns shine? They'll work in an overcoat pocket or as a IWB/OWB gun when you can wear an outer garment to cover it, plus I've read that like their smaller brothers they rule at weapons retention! Pluses: Big easy to use sights, darn near unlimited +P usage, and probably the smoothest action known to man! Minuses: BIG, and heavy.
.357 vs .38; Go .38! The magnum is fun at first but after awhile you (and I) go back to .38s just because they're easier to shoot and with the right ammo get real close too the .357! Sorry for the long post, I hope it helped though. Dale