Most modern revolvers use the pin in frame design. The idea being the pin is more accurately lined up with the charge port, and it lines up with the primer for a more direct and straight strike. It also allows the use of a transfer bar safety system, very much in vogue these days. Downside is no direct connection with the hammer, a floating firing pin that can stick or fail to return to position due to a broken spring or crud building up around it. You also can't see if the pin is damaged or broken.
As you can tell, I'm not a big fan of it, although it does work adequately.
Pin on the hammer: Easy inspection of the pin. Positive connection to the hammer, it will always travel with it, therefore not get stuck or fail to return to position. Less complicated, less moving parts to fail. It requires a hammer block safety instead of the transfer bar system, which as far as I can tell works just as well preventing accidental dishcarge. Downside: Pin is more exposed to damage if gun is dropped or mishandled with hammer cocked (not likely, but possible), Pin does not strike the primer straight, but at an angle. It does however strike with more force IMO.
As you can probably tell, I prefer the pin on the hammer.