Can't comment on the Flexi, but I and a few friends all have Air-S boards - mainly cos they're easiest to get round here!
However, they're great boards - loads flex - so much that I thought I'd snap mine (being 195lbs/15stone (weight provided in both formats for reader friendliness!)) pretty quick, but it takes all I throw at it and keeps on comin!
It's also withstood some very amateur downhilling from several weighty drunks on one occasion....
IMHO not having owned any other board, I love my Air-S. I'm only learning kiteboarding right now, but it's taken over 100 hours on a rough, pretty hard surface, several jumps higher than I would have liked, and the occasional sudden drop into a pothole which hurt me far more than they hurt the Air-S!
Only problem I see with it, is that all my friends own em too, so it's easy to get em mixed up - or was, til we all piled down to the sticker shop!
Re the Flexi - I love their shape, but the fact that they're solely for kiting put me off - I like the idea of having a board that can go down a hill too, although since I sprained my ankle, I doubt that it ever will!