Two very different cruises; each terrific in it's own way. Do you like cool weather, beautiful scenery, wildlife, don't mind a little drizzle? Try Alaska.
Do you like Caribbean sunshine, swimming and water sports, beautiful scenery, wildlife, don't mind a little rain in the afternoon? Do the Panama Canal.
Better still--flip a coin! Do whichever wins next year and the other the following year.
Having said all that, I'm basically a warm weather, water sports kind of guy--I would do the Canal; in fact I am--next April.