I went with a Gen4 G23 a year ago then got a Lone Wolf 40-9mm conversion barrel. Really the best of both worlds. I also have a Gen2 G22 and Gen3 G27 and like the earlier generations but would buy the Gen4 today. Not a big enough difference to sell the older versions and replace though, why take a chance when you already have a reliable gun that you trust.
One nice thing, the 9mm conversion barrel also works in the G27.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek