Part of the answer depends on why you chose the Eastern Caribbean instead of Southern Caribbean. If you chose Eastern Caribbean because of the ship, or because you want fewer port stops in the course of a week than you'd get on the more port-intensive Southern Caribbean itinerary, than I'd stick with your choice. If you have been to the Caribbean before on a land vacation, and really are making the ship your destination, then just plan to visit Antigua another time.
It may be more convenient (and cheaper!) to leave from Florida rather than San Juan. If that went into your decision as well, it's another good reason to stick with Eastern Caribbean.
Your itinerary also visits beautiful places, and since you can't hit the ports of the Eastern, Western and Southern Caribbean all in a week's time, and you have to make some choices, go with the bigger, newer ship. I can almost promise you this first cruise won't be your last!