well, techically I haven't done the 7 day (yet--33 more days to go!), but from what I have researched and read, the Eastern is more like your traditional cruise, with more day's at sea and tropical/island ports. A lot of people have recommended the Eastern for a first time cruise. I have also read that the shopping is better on the Eastern, while the Western has more excursions and ports featuring the outdoors and exotic activities (hiking ruins/diving/snorkeling).
I personally was more interested in the Western ports, but the dates didn't match with my vacation time. But I am just as excited about my Eastern cruise, and am really looking forward to the days at sea. I guess it comes down to what you are looking for--more time on the ship and onboard activities which is what seems to be the appeal of the Eastern, or more ports and onshore activities--the bigger appeal of the Western.
I have been to St. John though, and it is absolutely gorgeous--so to me, getting to go there again is a big plus for the Eastern. Since I am going with my grandmother I am glad we got the Eastern dates, since she wouldn't be up to the hiking the ruins in Mexico, or the diving and Grand Cayman--which really is what I hear most about. I think she will enjoy the shopping and island tours more.
I'll save the night life of Key West and Cozumel when I cruise again with my friends! Just means I have to take another 7 day cruise... darn it!