So, you want a half-lug? Good choice, all told the half lug is probably my favorite barrel configuration.
I have a Model 66, and just ordered a 620, so I can't really comment on that one yet...but give me a little bit. With the 620 you are limited to 4" barrel, and of course the lock is there. I have never dealt with the lock before so I can't really comment on whether or not it hampers anything mechnically, but I know it does take away from the aesthetics of the gun. The 620 is based on the larger L frame, while the 66 is based on the K frame. I personally really like the K frame and was sad to see it get faded out. However, I do already have a 686 (another L frame) and I really love it. With the 620 you do get the extra shot which can be a plus or minus depending on who you ask. Up until about a year ago I had never bought a used gun either...now I have several. I have been lucky (good enough?) to not get saddled with a dog so far. And I can say that nothing feels better than getting a good deal on a good shooting and good looking revolver from the used shelf. Sometimes you can really come across some good deals. So, between the two it is a tough call...especially since they are basically filling the same void. If you are really uneasy about getting a used gun, then your choice is the 620. I don't think you will be dissappointed with it, and I don't think I will be either. If you like the idea of an extra shot, and a bit more durable frame, go with the 620. But, if you can find a good deal on a 66, if you want something more small-medium than medium-large go with the 66, if you want a barrel length other than 4" you will have to go with the 66, if you can't get over the lock get the 66. Again, either way I really don't see you being dissappointed with either of the guns.