Rupes or Flex 3401.
Even the Duetto from Rupes is a pretty significant jump in performance. I personally can't see the advantage other the 21 over the 12 at this point. The shop I help out has the 21 molded.
The Flex 3401 features gear driven, forced rotation. It's kind of like a rotary buffer, but a lot easier for the novice.
They're definitely pro grade tools, and help in the journey to beautiful paint. They're both smoother, and quiter.
And to explain why...
Being realistic, I could probably do everything I need to between my Makita 9237, and Porter Cable 7424xp. With the Lake Country Purple Thin Foamed Wool, and the myriad of microfiber pads available on the market, the Porter Cable can reach pretty far into the correction territory in pretty efficient time. Obviously the PC can spin 5.5" pads with no issue, so in my book, that makes it a very versatile tool. No to mention that I can also scrub carpets with it too.
For those bigger/harder jobs, the Makita rotary with wool Meguiar's pads can handle all that, and be followed with the PC.
I got that Idea from Mike Phillips. Made a ton of sense.
The reason I went with Rupes, was because they're great innovators, and manufacturers. They control everything in house, and they made a high performance tool with a body style that I especially enjoy using (LHR12E Duetto). The first product I ever used from Rupes was their microfiber pads. From then on I was hooked.
It hasn't been all champagne and roses though. Many a time I thought I'd have to sell the machine. From not understanding that the anti-spin shroud needed to stay lubricated, from not understanding how to prime their pads, to excessive dusting due to overpriming/overloading their microfiber pads.... It too me a good while (cars) to get it down. Now that I have a good grasp, I've been using the system in both my retail business, and at a friend's production shop, and it's been a phenomenal difference.
I had the Flex, and sold it. However, I'm in the market for another one. Like Rupes, Flex builds their tools themselves, and does a great job with it. I always found the 3401 to be really smooth, and comfortable to use. A true blend of rotary power, and random orbital "safety" makes it a great choice for people wanting to bridge that gap of rotary and random orbitals.