There are actually three kinds of shipping they use: EMS, registered airmail, and just airmail. EMS is fastest, and the airmails are roughly the same (though plain airmail seems to be slightly faster). For small cheap stuff they'll use plain airmail, and for everything else, registered airmail. EMS costs extra except for a few expensive gadgets.
However, in general, purchasing EMS shipping is not worth your time because it shaves off maybe 2 days off the 10-15 day total wait. Plus, if they split the order up into multiple shipments, only the first one gets EMS. You'd think, or at least hope, that they would prioritize EMS shipments, but I don't think they do.