The 3rd gen have the finger grooves in the front of the grip and a light rail.
BUY the 2nd gen if he is offering it up for a good price.
The 2nd gen 19 is known for it's undisputed reliablity.
I have a 2nd gen 19 and 23 and both have fed EVERYTHING I have put through them without a hiccup. If you purchase the 19, check with Glock to see if it had the upgrades done to it. If not and you want the upgrades done, they will ship the parts to a Glock armorer close to you for free. They will NOT ship them to you unless you are a Glock armorer. They will also do the upgrades for free but you will have to pay for the shipping.
Good luck and if you don't want to buy it, give me the info on it, I will