We've done both and both are great.
We did have a problem on the eastern however - one that you probably won't have. We went on the easter when our oldest child was playing basketball. He missed two tournaments (both saturdays). He was not a happy camper for those first 2 sea days because "there was nothing to do" and he was pretty sulky about missing the BB. Once we reached St. Maarten he perked right up.
East:
*3 Sea Days which
DCL fills with lots of fun activities
*St. Maarten - great beach island with the bonus of gambling on the Dutch side and hanging out naked on the French side.
*St. Thomas - well St. John actually. Some of the best snorkeling around. Absolutly beautiful!! Ok, there is that early morning clearing of customs thingy but hey, you're up and ready to take full advantage of the day - especially since the ship leave port at 4:30.
*Overall the islands are more interesting here on the eastern.
*When we did this cruise, we were traveling at 20knts straight into a 30knt wind. Going up on deck 10, infront of the windscreen in front of the sports court was a blast - a 50knt blast of wind that is. I had a death grip on the 7 and 9 yo otherwise they would have flown away like piglet in "blustery day" (look, I'm a kite). Took the kids back and did the same with DW.
West:
*Key West: if you're into partying neked its the place to be. Our kids weren't to thrilled when we wandered around town. We stayed in town less than 1/2 hour. We did enjoy the snokel excursion here a lot.
*Grand Cayman: Some of the prettiest water this side of St. John. Great snorkeling and sting ray city
*Cozumel: Really good for scuba, pretty good for snorkel. Carlos and Charlies for adult libations on shore. The water between the island and mainland wasn't nearly as nice (kind of a green color) as EVERY other port we've been too.
*Only 2 sea days: and DCL didn't seem to have nearly the variety and quantity of activities as on the eastern. We spent the sea days here waiting for the next stop vs. the eastern where the sea days were a hoot in and of themselves.
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OK with a 3yo and 5yo I would suggest the eastern trip because you will probably will just go the a beach on your own and the beaches are very nice on the eastern. The sea days will be spent by the pool. If the 3yo is fully potty trained then s/he can go to the Oceaneers Club. The 5yo will probably want to spend most of the time in there because the kids have a blast.
Good luck making up your minds. And have fun!!