I think the short answer IMO is that at those prices, and if you use both kits stock, as they are, then the G25 is the better choice.
The G25 is a good, proven wheel. The build quality is nicer than that of the 911 Carrera. Good, durable pedals, responsive, large paddle shifters, and superior clamping systems for the wheel and shifter. At the price, it seems like a good deal, and you could sell it off and likely make a profit.
The 911 Carrera is more plasticky, and the standard pedals are merely adequate. I've found ways to mount my Fanatec wheels to a desk or table securely, but the mounting rods used to secure the shifter are pretty lousy. All Fanatec wheels have an awesome tuning system that allow you to adjust wheel settings directly in the firmware, which is a pretty huge benefit. If you're planning to spend more money down the line, the ClubSport pedals are an awesome, but expensive upgrade, and they only work with the PS3 via a Fanatec wheel. Those pedals are nice enough that I think they make a bigger difference than wheel choice in the $350 and under price range.