Here are my two cents:
You said you wanted it for the range for "informal" shooting. The 439 has slightly improved sights over those of the 39-2, so I would get that if you can get one at a reasonable price. I don't think it is worth $150.00 more than a 39-2, so if you are still looking at the same two guns I would get the 39-2. They feel the same in the hand and look almost identical except for the sights.
The 639 stayed current longer so there are some different trigger guard variations. If you go that route, get the one you like.
Depending on condition, I think $350 for a 39-2 may be a bit high, but not bad, and if it is particularly nice, not a bad price. But I think $500.00 for a 439 is high, unless it is like new, and perhaps even then.
A 539 is the same gun as a 439, except with a steel frame. They are less common, because most customers either would go for a stainless steel 639 or an alloy 439 and the 539 didn't sell well. So, you now pay a premium for their relative scarcity. I see little reason to pay the premium for a gun to shoot.
Beyond this, it is personal preference and pricing. There is little difference between the models. Oh, one last thing. I know there are exceptions, and some will disagree, but I would avoid the ambidextrous safety models with the ambidextrous safety models with the right side lever retained by a philips screw. They have a reputation for falling off.
So pick one and go shooting!
