My opinion only:
1. The 3401 is a PIA. It works - but it beats you up. Prolly good to work on some jacked up oxidized boats or something - but, who wants to work that hard?
2. The new Flex 15 (or what ever it's called) is an unknown quantity. To me, they are just jumping on the large throw bandwagon. Dunno why anyone would pick this over the original??? They don't make pads to support it. I'm sure it works fine - but I see no real advantage.
It's a new design - and it could have some bugs. Dunno about their customer service or parts availability. I'm guessing if it breaks after a year - you have to send it someplace to get fixed.
3. Griots 15/21 - people love this. Mainly I believe because of the warranty. Customer service is great, and they offer a system type approach. Pads and polishes seem to perform well.
But, I have seen some issues posted. Bearing problems, BP problems, etc... I would just rather go with the original, and what I believe is the best.
4. Rupes 15/21. They invented the large throw DA. They are tool manufacturers. They obviously come up with new technology (see iBrid) and update their product line as needed (see Mk II).
They IMHO, have had the LEAST number of real complaints with their machines posted on the forum. Only real "issues" I can remember was some people complaining they didn't like the silicone grease that came on the BP.
Rupes is a proven entity at this point. They may not have a lifetime warranty - but replacement parts are readily available. They have excellent pads and polishes. Can use same pad type and polish on the 15/21 as you can on the Mini and get the same predicable results. It's just easy.