Both are GREAT ships, with wonderful crews! We've done both of these ships multiple times, and honestly can't recommend one over the other. Needless to say, the Caribbean Princess is much larger, but that may, or may not be a plus. I think "Miracle" may well have a better kids program. Bottom line, I think I'd base my decision on itinerary, and price.
(We're doing "Miracle" again in November, to the Southern Caribbean. Can't wait, but I'd do the Princess again in a heartbeat as well.)
"SKY"