I have both, I upgraded from an old craftsman 8 or 10" orbital to the PC using 6.5" Lake Country CCS pads...........huge step up. I bought the flex 3401 about a year ago, and it is definitely at another level if you need something that will remove scuffs, scratches, or just need something that will do a better job with a cleaner wax. The biggest difference in my opinion is the forced rotation on the Flex, the PC will stop spinning if you apply too much pressure. If you don't have the issues above to worry about, the PC is works perfect in my opinion.
The Flex will walk on you a bit until you get used to it, but that's pretty minor in my opinion.
I'm a hobbyist detailer, but have been anal about my vehicles since the day I got my license..........far from a professional. I don't regret my Flex purchase one bit.