I'm not expert, but I think I can give you a start:
Python - mostly .357, but may have come in .38 only. Considered the best of the bunch. Old mechanism that is smoother than the newer ones, but I'm told is more susceptible to breakage. Comes in various lengths from 2-8 inches, and in various finishes including stainless, nickel, electroless nickel, and blue. I have a 6 inch nickel, and love it.
Anaconda - mostly .44 mag, but I think they came in Colt .45 too. Bigger than the Python. Comes in stainless only, I think. Most are 6 inches, but I've seen some 4 inchers too . Mine is a 6 inch stainless, and it's nice but not as smooth as the Python.
King Cobra - .357. This is considered the poor man's Python. Don't have much experience, but I know it doesn't have the vented rib above the barrel like the Python does. I've only seen 4 inchers in stainless.
Diamondback - Know the least about these. I think they're .38 only, but not sure.