Not that it really matters, but since you're already familiar with Griots machines, the G15 will keep you "in the family" so to speak.
Performance wise, the G15, like the GG6 is a random orbit machine, and will probably be much more comfortable for you to use, while still delivering increased performance capabilities from the GG6.
The Flex 3401 is a true dual action machine, where gears placed in the spindle housing, and on the back plate eliminate the free spinning effect found on GG6/G15 type machines. This machine can handle downward pressure, allowing for increased cutting abilities, they also polish, and wax effectively as well.
Transitioning from the GG6 to either the G15, or 3401 - I would imagine you would have a much easier time transitioning to the G15. Not the the 3401 isn't an awesome piece of machinery, it just takes some getting used to by comparison.
You can relegate the GG6 to be used with smaller pads and/or brush attachments if you desire to keep it.