I've done both, but MUCH prefered the Western. Like several other posters said, it depends on what you like to do on a cruise.
Western tends to be more "active" - Eastern more laid back and better for shoppers.
We loved Belize, loved Costa Maya and were pleasantly surprised at how much we liked Cozumel. Could have skipped Grand Cayman entirely, IMHO, but lots of folks disagree.
Eastern offered up St. Maarten, which we loved, but I could live without Nassau and Freeport.
Carnival Elation March 11
Carnival Imagination Sept 07
Carniival Sensation Dec 06
RCI Sovereign of the Seas Sept 06
Carnival Miracle Sept 05
Carnival Glory Sept 04
Carnival Fantasy Jan 04