Skip the G-6. Fly the G2. It has more ammo, flies faster, climbs better, turns tighter.
The G14 and K4 are fairly close performance-wise up til about 16k or so. After that the G14 drops and the K4keeps going. I'd suggest the G14 only because it has more weapons options than the K-4. If you can fly one, you can fly either, so go back and forth as you see fit.
The G6 gains a lot of weight and loses a lot of performance. Historically it had its uses, but given the choice we have, I say avoid it entirely.