The difference is actually $200.
The Flex does not work as well with microfiber pads (I dont like it anyway.) This may be real important to consider. The new MF pads are revolutionary. I am using them more and more. I prefer them over foam in most circumstances. They correct faster and clean on the fly with just compressed air. I machine wash and dry them when done.
The Griots is easier to learn with and more forgiving. I think this makes it a better choice for people new to machine polishing. It might stop rotating on some curves, but, that is what makes it safer to use.
For an experienced user, where paint safety is not as much of an issue, the Flex is faster to use. I dont like it as much as it tends to walk and grab more.
So, my recommendation is to buy the GG now. Learn on it with foam and MF pads. When you get more proficient and comfortable, buy a Flex. You will have 2 great tools. Each have their strengths/weaknesses. Plus you will have a spare should one go down.