I use my MX500s far more often than my PortaPros for two reasons:
1) They leak a hell of a lot less. Like, a HELL of a lot less. I listen in class quite often and that's a big deal, because I can raise the volume higher.
2) As a result of listening to my MX500s so often, the treble on the PortaPros sounds dull, recessed and yucky. Granted, they go much lower than the earbuds, and have a (bit) sweeter midrange, but that treble is just gorgeous. It's detailed and non-fatiguing, and, for a $15 earbud, the soundstage is nice and big.
I love my MX500s and couldn't possibly live without them.