Current 66s have been slightly beefed to hold up better than the old ones, which would wear badly with steady magnum use. One place they strengthened was the breech end of the barrel, which used to take a real beating from fast jacketed bullets, sometimes splitting. If you go with an older one, check that area for wear. Really, those guns were designed for practice with .38s and duty use of .357s.
Also consider one of the L frames. They're a bit heavier, but much more robust and are available in 7-shot versions (which actually have thicker cylinder walls than 6-shots because the notches are offset). I picked up a like-new 4" 620 for $350 when my son worked the gun counter at Gander Mountain and it's a fine piece. Some don't like the two-piece barrel setup, but that's what they use on the .500 and .460 without issue. Most of the current Ls have conventional barrels I think.
Nothing like the single-action trigger on a Smith. Both of mine go off almost with a thought.
