Its really up to you. Do you need space, or could you go with both a coupe or sedan. Lower miles is always better.
There are cars everywhere, g6's everywhere. I suggest you get the exact color you want, and the one with the least miles you can find. There will always be another car, so look until you find "the one."
I have an '06 gt sedan in black, and love it. I also have a pano roof. haven't had really any problems, except it can get a little noisy on the freeway.
2006 G6 GT
Mods so far: Black arrow overlays, Chrome recessed grilles, Blue LED license plate light, Blue LED reverse lights, Smoked tails, 20% Tint all around, Red led's for the interior, White led dome and map lights.
Sound System: 4 Infinity Kappa component speakers, 2 Amps (950w total) and 2 10" Kenwood KFC subwoofers, 7" touch screen Pioneer DVD
Performance: Mobile 1 oil, Mobile 1 oil filter