I would go with the Flex over the MT300 hands down.
With the forced rotation you will never have to worry about the pads not spinning.
The flex and handle 5.5" and 6.5" pads with no problems.
The build quality of the flex is a,asking and you can feel that it was well put together IMO
The Flex was made to polish paint and that all.
While some say the forced rotation can be a bit difficult to get used to I was able to get a feel for it in a matter of minutes when I first used one.
Additionally If you go with a flex I would get the longer cable if they make a 220v option.
I used a Flex on Air Force One for 16 hours a day for 7 days when our group did it and we all had the Flex