Personally, I'd go with J-mo if I'm looking for playful. I'm one who likes a more wide and stable platform. Both have similar rocker, the J-mo is just a little more better as an all around sort of ski. Despite riding every year of J-mo and Filthy, I've not spent an entire season on them, so someone who's owned either should chime in here on how they are to live with.
"He was the spearhead of the mod force. Everyone knew who he was and feared his wrath. There really isn't anyone like that anymore. Rowen didn't just delete their threads, he deleted their souls."
I have both and I use both in the park, most of the time my jmos are used for presses, bonks and slower/smoother spins (think Kieren McViegh) while I use my FRs for more technical tricks (swap combos, bigger spins, ect.)...both can do both (although it is slightly harder to whip the jmos around but once you get used to them you can do it), that's just how I found myself using each ski. Honestly, I found my jmos being my go to ski if I wanted to rip around out of the park and then pop in every now and then, they're certainly a more stable ski than the FRs (not that the FRs can't handle going fast, just the wider platform on the jmos gave a more stable feel), but if I wanted to just focus on new tricks in the park I'd be on my FRs.
To answer the flex question, the FR is slightly softer than the Jmo, but both are really just as playful (I can butter one just as easily as the other).