From my experience,
If you talk about the full blown slalom boards.
The Falcon sails looser than the Isonic.
This makes the board more nervous. Some people like it and think they go faster with it.
I like the stickier feeling of the Isonic, which makes me more confident, resulting in higher speed.
The Falcon gives me the feeling like it wants to fly off the water.
It's definitely less controllable in chop.
Top speed of the Falcon is probably comparable with the Isonic, if you know how to control it.
I think that the Falcon is a more technical board.
I'm talking about the Falcon 2009.
It seems that latest Falcons should be easier and not that loose any more.
IS87 / IS101 / IS122 / US147