Sableye is primarily a zombie like, physically stable creature, thus primary Dark and only secondary Ghost. Spiritomb is an evil spirit, tough physically unstable, thus primarily Ghost, secondary Dark(evil)
Wow this entire time I thought they were both Ghost/Dark. >_>
Which I didn't like because Dark/Ghost rolls off the tongue easier...and sounds cooler (it even makes grammatical sense). --- In the age of information, ignorance is a choice.