fmattox has been there and has given me advice- he is currently cruising Hawaii right now...so post this again next week and he can give you some insight.I pulled up an old post I did in the General Cruise Chat Forum you can look there.
Anyway I know when I've looked into it I was told...go with Princess, make sure you go "through" the canal not just Miami port to leave and return Miami port. I look at not the total number of days the cruise is, but how many days we're in ports vs. out to sea. I don't want to be on the ship a lot of my days. We enjoy doing and seeing at the various ports...we don't do well relaxing by a pool. But that's us. Also the timing of the Panama Canal trips...it's not like they're offered year round. We already have a big cruise planned in Oct so we can't afford to do another trip in Jan or March. So we may have to go Oct or wait til '09.
That's okay...whenever we go ... it'll be great!