Gonna need a little more information to help you out. Let's start with..
-what polishers, if any, are you using now?
-what are the most common details you perform? (2 steps, 1 steps or all in ones)
-what condition are most cars on average that you're working on?
-lots of curved body panels or tight spots?
This might help others to help you.
I will say after using the griots and rupes this past weekend they are extremely similar, but if you choose either of these, its best to buy into the whole system. Whether rupes or griots BOSS. These products were all developed to work together to achieve maximum results during all stages of the detail. They are smoother in operation to the Flex with minimal vibration but can stall relatively easily in tight spaces or curved panels.
The flex is a totally different animal and as Mike Phillips calls it, "The Beast!" The polisher won't stop until you turn it off, this is great for curves and can be tilted on the edge of the pad for tighter spots. A con related to this though is the polisher "works" you and can "walk" on you if you're angling even slightly and not aware. Idk I may be in the minority but this polisher was a bit much for me and not too sure I'd want to use it daily. I'd bet this would knock out cars quicker than the 21 mm DA's though, that's why condition would matter too. If the cars are wrecked and you need quick turn around, you might want to get familiar with this tool!
-David