I like the 19 better than the 17, perfect size for me. I'd rather have Gen 4 but nothing wrong with Gen 3, try both and choose the fit you like. What I have is the 23Gen4 actually and a 40-9mm conversion barrel.
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