If your going with the 29cc engine I would go with a 700, one of the guy's on Helifreak went with a 30 and had to go back to the 26 (the 270 is actually 26cc, it's called a 270 simply to signify the use of a 4 bolt head which is a must on a heli, do not use a 2 bolt head engine) as you correctly statehe had prbolems with it eating tail drive gears.
It's a pointless exercise going with a 600 and then changing the boom and blades to end up with what is basically a 700, you may as well start with a 700.
If you haven't bought the engine yet don't dismiss the 600, I had both and the 600 with the 20 tooth pinion and stock G270 was fine. The 700 was however in a different class for eating up the sky, the 600 is probably better suited to 3D. f youcan only have one and your not into really hardcore 3D then I'd advise going with the 700.