Both are pretty nice. Honestly it might be which you prefer to drive/which you prefer stylistically inside and out or like more.
Personally I've always loved the 6GC. Timeless and elegant inside and out, it reminds me of the perfect GT car if you need 4-doors. While the interior is "older," I wouldn't say the G30 has a modern interior either other than the new iDrive 7 and Gauge Cluster - which many argue is a step backwards from the previous hybrid analogue/digital gauges.
The 6 will be lower, wider, and longer, but if your back can cope with getting in and out its still a very nice place to be. Driving wise they both likely don't have many differences. The 550 is faster, but both are still plenty fast for everyday use and will likely never see a drag strip/track. The G30 is newer, but I doubt the changes to the driving dynamics are radically different to that of the 6 to sway a decision based solely on handling.
Also a quick note, AWD 6-Series models receive hydraulic steering, which is super crisp and delivers great feedback compared to modern BMW EPS.
Personally, I would take the 6GC and use the extra cash saved by buying used for warranties/savings, but thats mainly because I LOVE the looks of the 6GC and would want the hydraulic steering, and it just feels like a more special car. Try both out, whatever car you get I'm sure you'll enjoy it!
