You've come to the right place. Here's where to start:
http://www.thehighroad.org/showthread.php?t=261635
I have a 39A made within the last year, and a 39M from 1973.
There are two major changes:
1. The new one has a rebounding hammer.
2. The new one has a hammer block safety.
The quality, fit and finish are identical. The walnut on the newer one is even nicer than the old one, and has excellent cut checkering which the old one does not. Other gun companies might sell the walnut on my 39A as "semi-fancy".
Unlike too many others, Marlin didn't get the memo that new guns are supposed to be slipshod, poorly-finished, with crap wood (unless you pay "custom shop" prices). There are many guns out there that are so-so if new, but great guns if older production. The 39A is NOT one of them. The new ones are top-notch.
So, that leaves the rebounding hammer and the safety, which are on the new ones, but not old ones.
I like the rebounding hammer. It's an automatic safety that holds the hammer at half-cock and locks it away from the firing pin unless the trigger is pulled; it's useful for hunting where you might want to go through the brush with the hammer down and thumb it cocked if you see a rabbit. However, the old-style down/half-cock/full-cock shoots bullets, too, some say a bit more accurately, but I can't confirm or deny that.
The crossbolt hammer block manual safety is another matter. Some people hate it. Others see it as a good tool, since it lets you decock the gun without any danger of it going off.
The reason people hate it is that it doesn't block the trigger, just the hammer from hitting the firing pin. That means that, if you don't remember the safety is on, you can see a rabbit, cock the gun, pull the trigger and you get a frustrating "BING!" as the hammer hits the crossbolt instead of the firing pin.
As for me, I see it both ways. I think it's good that I can drop the hammer 100% safely, but I have gotten the frustrating "BING!", too.
My opinion, though, is just don't use the safety while actively hunting. Just use the rebounding hammer, and use the safety only when decocking in a situation that requires absolute safety, or topping off the tube. For target shooting or plinking, it doesn't matter anyway. Paper and cans don't run away.
I love my new 39A, and I wouldn't hesitate to recommend a new one. Like I said, the wood is better than ever, and the fit and finish are top-notch.
Beware the 39 Club, though.
Their influence led me to get my old 39M Octagon when I saw it in the store (a very rare model, about 2000 ever made, so I had to decide quickly if I ever wanted it).
If you get a 39A, it may well be your first one, not the only one you ever get.
) it is no big deal.