So religious arguments aside, consider the facts;
1) the 17 and 22 are the only models currently available as Gen 4's.
2) Gen 4 offers ambidextrous magazine release.
3) Gen 4 offers the ability to use 3 different grip sizes.
4) Gen 4 has a different grip texture
5) Gen 3 will be cheaper
So you're looking for a G20, if you want it now, there's no Gen4. Fact 1 dictates Gen 3.
If you're right handed, you don't need the ambi mag release. Fact 2 is a mute point.
Know what grip size you want. G20 is a large frame gun. You can get the G20SF (Short frame) and have a smaller grip. I have massive hands and the grip options on the Gen 4 made my mind up. I actually shoot with the smallest grip available (no backstrap on a Gen 4) to avoid Glock bite. If the large frame G20 or the short frame G20SF fit fine, there's no need for the Gen 4 (unless perhaps you're sharing the gun with a significant other with different sized hands).
Texture is texture. Hold on to your freaking chunk. If it slips out of your hand you're doing something wrong.
Gen 3's will be cheaper as people trade them out for Gen 4's (see the iPad comment above).
There are a couple other differences with a Gen 4 (like the dual recoil spring) but the change is so negligible it's not worth noting as a decisive factor.
There, facts, no religion. I've put far more than my fair share of rounds through many Gen 3 G22's, I've also owned one. I own a Gen 4 G22, and it sits right beside my Sig 226 Elite for the other haters
Hope that helps?