Aha! I can answer BOTH those questions!
Ferrari World - we went when it opened and found it dire. The Formula Rossa (fastest coaster in the world) wasn't working, and back then they didn't publish which rides were/weren't working. It is supposed to have improved since. If you've got a stopover then definitely check it out, but I don't think it's a park that would keep you entertained for long (though this is from our experience, where out of 4 thrill rides at the time, only one was open). There are other parks in the UAE to do (can't recall which but a new one has just opened), and getting to Ferrari World is an hour's drive from Dubai to Yas Island in Abu Dhabi.
PortAventura vs Disneyland Paris... hmm, good question. You say you're already in France. Basically, you should probably know what a Disney park is like. PortAventura is probably more like a mix between Universal and Six Flags, in that they've got nice theming and good, big thrill rides. Can't really help you there as the parks aren't similar enough. Do you want a Disney park, with all that entails (not many big thrill rides, though Aerosmith and SMM2 are pretty mean), or a more extreme park? Both may well have long queues depending on when you go.