Get a Griots GG6. It has the power to spin 6.5" pads, and has the versatility to use pads as small as 3". I don't believe a Flex or Rupes can use pads that small.
When using my Griots, I may jump from a 6.5" pad to a 3" pad, depending on the area I'm working on. Using the right size pad really helps. You'd be amazed how many areas require 3-4" pads on newer or sports cars. Also, sometimes, like on hoods, using a 6.5" pad can speed things up if your just doing minor correction or applying glaze or wax.
The advantage the Griots has is that it has the power to easily spin large pads. The other two machines (PC / Meg's) tend to bog down easier. The Griots even has the power to correct glass with the right pads and polish. A PC may be more reliable, but the Griots does come with a lifetime warranty - others I believe is one year.
Using a different size pad is as easy as unscrewing the backing plate. I was a PC user for years, but I switched to a Griots when they redesigned them and were selling them for $99. I'm very happy with it. And it's smooth and powerful for me.
If I was starting all over, I would want to start with a Rupes. However, correct me if I'm wrong, but I don't believe the 15 or the 21 can be used with a 3" backing plate (none made ?) - and that would be a deal breaker for me. I use 3-4" pads 40% of the time.
As for the Flex, I've never used it. But if your doing this has a hobby (occasionally) and are just starting out, for the price of a new Flex, you can get a Griots DA, multiple backing plates and pads, polishes, etc... - everything you need.