I prefer the K101 to the PX200. As might be expected from AKG, it has wider soundstaging, which helps make the portable experience sound a little less constricting than some other smaller headphones or earphones might. More importantly, however, it's a more neutral headphone than the PX200, which exhibits some mid-bass prominence that I find a bit much (I prefer the PX100 to the PX200). As often is the case, I think what kind of music you listen to makes a difference here, as someone who listens mostly to hard-driving rock, electronica, or rap might prefer the heavier bottom of the PX200. I listen mostly to jazz, movie scores and classical, so, between the K101 and PX200, it's the K101 for me.
Keep in mind that the K101 is semi-open, whereas the PX200 is closed. Also keep in mind that the K101 isn't nearly as portable as the PX headphones -- the PX headphones are smaller, lighter,
and they fold up. The K101 is hardly any more compact than full-sized headphones (sort of similar in size/shape to the HD497), but it is quite light (160g versus the K271 at 240g).
I use the AKG K101 and Sennheiser PX100 with my portable rigs when I don't feel like using an earphone, and, between those two, it's a much closer call, with each getting about equal time with me. I'm actually listening to the K101 as I type this.