We have done both the Eastern and Western 7-night itineraries and are doing the Western again this fall - we missed grand cayman on our last Western (2004) thanks to hurricane ivan.
I actually prefer the Eastern - you have an extra day at sea and St. Martin is just lovely. For some reason both St. Martin and St. Thomas struck me as being more exotic than Cozumel and Costa Maya (although Costa Maya was very pretty and very unspoiled when we were there).
We've always cruised in October and just roll the dice with hurricane season - just be sure to get your
trip insurance! It's a beautiful time of year to cruise.
If you have any other questions feel free to ask - you can also take a look at my October 2005 cruise report (it's in my sig.) if you want to know more about that itinerary. It's long...but would you expect anything else from me?