I owned both the PK3 and the RE2 (my daughter still has the RE2), and I think from a use POV the RE2 is a better choice, mainly because the PK ear buds could be a little large for most ears, and they tend to loosen up very easily if you are on the go. Plus, they do suffer from a significant amount of outside noise. I don't think the PK lineup is inferior SQ-wise, just from a functionality POV.
Having said that, I listened to the RE2s the other night, with a FiiO amp and my daughter's iPod (testing it, since it fell in the sand), and with the right tips and amped, the RE2s really do sound good. I put some Comply T400s on them, for a better seal than I get with the silicone single or bi-flanges (either size). Anyway, that adds cost and convenience too, as the FiiO amp is $8.50 (not much, a great ebay value) and the Comply tips are $20 for 5 pair.
Not sure this helps, but from a pure sound perspective, unamped, the PK2s are your best bet. But from a portability POV, the RE2s might be best.
Also, you could consider the Denon 551s, which are a little less money, but sound pretty good. They are canalphones, like the RE2s.
For more on the FiiO,
http://www.anythingbutipod.com/archi...ini-review.php