Good questions and I can only answer as honestly or truthfully with the limited time I have spent with it so my evaluation thus far is based on limited experience with it. I will have the machines up against each other on hard paint in a couple of hours but for now I will comment on correction done on soft to medium paints.
From the first time you hold the Mille in your hands to the first time you power it up and do your first section pass you begin to realise alot of thought, engineering and attention to detail has gone into making this polisher. It feels lighter than the 3401, as well as more balanced. Even the speed dial has an expensive feel and click to the different speed settings. The polisher is noticeably quieter and much cooler running than the 3401. Using the 3401 and Mille side by side you can feel the difference in vibration when going back to the 3401. The Mille doesn’t wobble as such and feels very well thought out, sophisticated and much newer.
Throughout the speed settings you can see it has more rpm at every setting and doesn’t walk nearly as much as the 3401 either. However if the pads not perfectly centred I found the Mille to be a handful to control so much so that I had to switch off and figure out why that happened. I have used many types of non Rupes pads on it as I have alot of them and donot intend investing in all new pads specefic to the Mille anytime soon. Having said that I find the intended Rupes pads or thinner pads to feel more natural which is the opposite of what we are led to believe gear driven polishers prefer.
Yes its sleeker,fancier than the ageing 3401 ofcourse, yes its smoother, quieter, and runs a heck of alot cooler too but bare in mind theres a decade technology difference between them, 2007 3401 vs 2017 Mille, was never gona be a fair fight at all.
Lastly correction..., the Mille is definitely the more powerful tool and faster correcting of the two, theres no second guessing that, it is what it is. I had to drop to speed 3 on the Mille where the Flex needed speed 5 and then too the Mille still had the edge. I used the same pads and compounds to keep things fair and at one point even put the more aggressive pad on the 3401 and then too the Mille was still correcting faster.
I hope this answers your question, also I will update again soon on hard paints and then know for certain.