Dude, the MX-300s and Apple buds are both awful. I gave both one listen then threw them in the drawer of crap phones/buds that came with players. Who cares which is less awful? You spent a bundle on a player, at least get some decent buds or phones.
The 400s are a huge step up. The 500s may be a slight step up from there, but AFAIK all it does is add the in-line volume. Neither are as good as the MX-4 buds that Sennheiser used to sell, or even vaguely close to the PX-100s (but those are headphones).