They use the same drivers.
The PX100's are open while the PX200 are closed. So the 100's are better for indoor (and areas of low sound concentration) usage, whilst the 200's, being closed, can deliver sound better while you're outside.
It's a choice of preference mostly, but the 100's do have a slight edge simply by the virtue of being open.
The only downside to this series is that they sit on your ear. If they made a circumaural version I'd buy it so hard.