I have both and use both. Like anything else, they both have their place. The 21 is awesome to use on flat panels, a joy to use as it is very smooth, and doesn't require much pressure to correct.
The 3401 is great on curved panels, or anything with a contour. The rupes still stops for me on anything with a curve, and yes I use the washer mod. I've tried my technique many different ways on curved panels with the rupes, and while I can get it to rotate, it doesn't rotate consistently and still wants to stop. Put the 3401 on it, turn it on, and it's going to correct without having to think about it, period.
Both are great machines, and I enjoy using both. I personally have never had the issues with vibrations like others gripe about with the 3401. I like using my flex, and like I said, I like knowing that if its on a panel, no matter how's by curves, it's going to spin.