The G3 is supposed to be the successor to the G3. But the only improvements lg made were in design. Although still plastic, apparently cheap, brittle plastic based on recent reports of cracks in the G3 frame, the G3 design looks more premium. It has a better battery cover although it loses points for trying to fool the public into believing the rear cover of the G3 is metal, when in fact it's plastic with a metal mimicking paint finish. Just like Samsung did with the faux leather battery cover on the Note 3. If you can't use the genuine premium material, don't insult the customers' intelligence. The G3 improved the rear buttons. They're much easier to find and utilize. The G3 screen is a half inch bigger. That's negligible. But it's bigger.
Unfortunately that's when it all goes down hill. Where the G2 is a battery sipping, buttery smooth operating speed demon, the G3 is beset with a laundry list of bugs, flaws and defects. OP, you already alluded to the complaints you've heard about the G3. No need to rehash them. Just scroll through the threads. Lg just announced a major update for the G2 coming at the end of the year.
G3 is the pretty girl in school that nobody hangs around with for long because her personality sucks.