I've owned a Carvex. Which was a blind purchase, I thought hey it's Festool it's gotta be good. It wasn't.
Can't attest to the newer 420 model, but I've seen some (mostly here on FOG) positive reports on it. I'm still sceptictal though. The first Carvex left a very bitter taste.
I've now got the P1cc for almost a year and I must say I like it. So in my opinion, it's a no brainer to go for the Mafell. You can get it with all the accessories in 1 go, in a single systainer. No need to buy a seperate accessories systainer. So in the end, the price difference isn't all that big, at least not over here.
The only reason I can think of to go for the Carvex is you can't do without a plug-it lead and maybe servicing, but I've never had to use servicing so for me it's not part of the equation. If you have any specific questions, just send me a PM.