Dave and Carrie, as Susanmax indicated, it really depends on your expectations. Both are great ships, it just depends on what you are more comfortable with. I wouldn't discount the Norwegian Star from Seattle either if you are younger and looking for a more unique dining experience.
Tough decision, but they are all great. The worse case scenario is that you will still have a great cruise. BTW, May is the very best time to cruise Alaska in my opinion.