My first polisher was the Flex, I wanted the best I could get in a DA and it certainly earns it marks. I think of the Flex in a different league than other DA's (inbetween the PC and Rotaries), my uncle and I did a comparison on the PC and the Flex, the Flex broke down the polish faster, plus it does not create as much noise or vibrations (the PC has a ton of vibration) If you have the funds to get the Flex go for it!
The only gripe is the inability to change the BP, but that is what a second polisher could be good for, whether a PC or other DA or a rotary. Even with my Metabo Rotary I still use the 6.5" pads, but I can change them out when I need to do smaller areas.
Let us know what you decide!