It depends entirely on you. I saw try and shoot both and go with what you like better. If this isn't possible then I'll say the Gen4s are a little bit of a crap shoot right now. I'm buying one but I have plenty of Gen3 19s so I can aford to test it before I make a decision/feel comfortable with carrying the Gen 4. You might be better off buying an older or used Gen 3.
I have a gen 3 and gen4 g23. I love them both but not enough difference to me to pay the the extra bucks for a gen 4 that may or may not have problems. I have to say that when I took the gen4 out for the first time, the very first shot I fired failed to eject the brass. It takes a lot of shooting to trust a weapon after that happens. I had a couple of other minor problems early on but now it shoots great, good enough that it is my daily carry. The only advantage might be a better fit for a ladies hand with the different back straps but honestly there isn't that much difference. This is not a bash against gen4's, in fact, I have a gen3 g19 and at some point, I will buy a gen4 , not because it is a better gun but because I don't have one. Hope this helps.
If the Gen 3 fits your hand then buy it, if you like the back strap change of the Gen 4 to fit your hand, buy it. My nephew has them both, he said don't buy into the better feel of the Gen 4 recoil over the Gen 3. He claims its not enough to tell a difference for worth paying more for the Gen 4. So its up to how it fits your hand, is his opinion.
He had no choice but to buy a Gen 4 his Gen 3 had the rear frame rail failure that was in the 2001-2002 models. I don't remember the months right off hand but I think it was Sept. of 2001 to June of 02 give or take. So when he got a new issue gun it came as a Gen 4. Glock did replace his frame on his Gen 3 but he does carry the Gen 4 on duty.
My G26 is a Gen4. I love that the small backstrap makes it perfect for my small hands. I'm glad I don't have any of the reported problems of the Gen4.
My brand new two-week old G17 is a Gen3. Before buying, I shot both the G17 Gen3 and G4 side by side using 50 rounds each, 10 rounds at a time, alternating between the two. I went with the Gen3 because I loved that the grip doesn't hurt my hand after shooting many rounds. The RTF2 on the Gen4s are very rough to the hands because they are made specifically for the military and LE personnel who wear gloves in the field. In addition, I had quite a few shell casings hit me in the face and chest (with a couple even going inside my dress) on the Gen4. This happened despite being a newer Gen4 with the better extractor. And no, I was NOT limp-wristing it!!! So... I had a much better experience with the Gen3 G17 than the Gen4 which is why I chose to go with the Gen3.