Either is great; my preference is for the Gen 4 because I like the built in beavertail option in the backstraps. Without a beavertail on my glocks they bite the snot out of my hand. The only real problem with the backstraps on the Gen 4 is that some of the aftermarket magwells will not accept the backstraps without modification. It isn't a big deal, I simply cut the bottom of the backstrap off flush with the top of the magwell. I then use double sided tape to attach the backstrap to the pistol. It isn't elegant but it works just fine.
If the Gen 3 fits your hands, then I'd just go that route and save yourself a bit of money and headache. If you run a Gen 3 and want a beavertail, the gripforce are cheap and work well.
My Gen 4 has aftermarket sights, guiderod, zev ultimate trigger kit, zev magwell and arredondo mag extensions. I'd say the most worthwhile mods were sights, magwell and mag extensions. Everything else is just fluff.
"I wanna go fast" -Ricky Bobby
"Mind bottling isn't it?" -Chazz Michael Michaels