We have done the "regular" Eastern, a "modified" Eastern (due to a hurricaine), and the Western. Of the three we enjoyed the Western the most. We LOVED Cozumel and Grand Cayman (hard to say which we enjoyed most) and Key West was great fun, although that had more to do with the group of people we were with then Key West itself. We had a "Savanger Hunt" with about 40 fellow cruisers and had an absolute blast!
All that said...it could be we loved it so much because we were looking forward to it and planning it, for 2 years (since the 1st Western we booked , turned into an Eastern).
In Ports we have done:
Eastern: St Martin/Dawn Beach
St Thomas/ 5 Star Snorkel Trunk Bay
DCL trip
Modified Eastern: Bus tour in Prurto Rico
St Thomas Trunk Bay (on our own)
Western:
We swam with the StingRays with Capt. Marvin in Grand Cayman, and booked a private Snorkel/dive trip with Eagle Ray Divers in Cozumel.
We did re-book the new Eastern cruise with Antiqua, but if Disney had not offered a new port we would have re-booked a Western.
Do some research on the Trip Repot boards and on the ports offered on each cruise. The cruise itself is almost identical, the ports are the only variable.
No matter which you pick you will have a wonderful time, then book the "other" cruise before getting off the ship!
Also keep in mind that the ports you visit ARE subject to change depending on weather and port conditions.