Sure, I have both and would never be without any of them.
The Flex is a beast, paired with 4" backing plate and 5" Hybrid pads it's the best companion to a Rupes 21.
I say this because although the Rupes works wonders, there are some panel shapes and sizes it won't fit perfectly. That said, it's likely you get uneven results at the end, on a higher or lesser extent, depending on vehicle's design.
This will never happen if you have a Flex for those areas. In fact, I may work a panel with the Rupes, and 'redo' just after the parts where it 'struggled' with the design using the Flex. It's very fast and easy to do if you have both tools ready to go, and will get your results to another level.
Otherwise, I may cut with Rupes and Microfiber and finish with Flex and Foam, or the opposite (Flex with cutting foam or even wool, finished by the Rupes)... Combinations are endless.
They are both at the TOP, but each tool complements the other. When you have lands of flat and large areas, the Rupes is King. You will save time using it.
When you have complex curves and designs, Flex is King.
Whenever you have both tools at hand to get the job done, YOU will be the King my friend, go for it, !
*PS: My 'other me' knows that Flex will be releasing something new the end of this year. That said, I'd wait a bit for more info because if they offer something even better than 3401 (how's possible?!), it may be your best choice.
Just to keep the eye opened.
Kind Regards.