I have no idea of the percentages for each....both are still good in September of this year....but you are going to need passports to do any travel anywhere outside the U.S. borders in the near future so I strongly urge you to take the plunge and get the passports. Why let your future travel plans be encumbered by not having passports? They are good for 10 years for adults.
Another thing to consider is that if you have to leave the cruise early and fly home to the U.S. you will need a passport to fly home. You never know if this might need to happen.