If you want any chance of having the two cabins near each other, you really need to book now. Plus, as mentioned above, the early saver guarantee protects you in case the rate goes down.
I have cruised many times around Christmas and it is so neat to see the Caribbean ports all decorated for the holiday. Just seems a bit out of place to me since I grew up in Iowa where it is always freezing cold at that time of year.
Have a great trip!