You can make the reservation 11 months from the date that the cruise is scheduled to sail. Notice that it's different from regular DVC reservations that are from check out date.
We've done both and prefer the Eastern only because you get an extra day to relax at sea. We also like going to Anguilla, which is where we go when the ship docks at St. Maarten.
I think the best way to decide on your itinerary is by deciding which ports of call you would enjoy the best. You'll have to do a little homework. Best place for that is over on the DCL discussion board and your search bar.