The D6 is a very fast, under stable driver. It is designed for all players and flies extremely far. The D6 will flip up when thrown hard, turn towards the right (for right handed throwers) and then go into an extremely long glide, with a gentle draw finish to the left. The D6 is also a great disc for long controlled rollers.
The 400G series is the regular 400 plastic with a shot of adrenaline. It is the base 400 plastic mixed with another resin that give the plastic a slightly different feel than the base 400 series. The additive also makes the disc more durable and requires more time to break it in.