Hi Jack Stephan Sr., welcome to CruiseReviews! What a fantastic choice. We were scheduled to do the canal a couple of years ago, but couldn't go after the economy took a nose dive. I'm glad you're getting to go!
Either cruise line is going to give you marvelous service, great food, and comfortable cabins. Princess is my personal favorite, but that doesn't mean it would be yours. Both lines are elegant, not as many announcements, and they don't push activities down your throat. I personally think it boils down to the itinerary and price. If one offers longer in a port than the other, or the price is more reasonable with one, then I'd go with that.
We've sailed both HAL and Princess, so for me, I'd book Princess unless HAL was a good deal less than Princess. So far, the prices I've seen, they're pretty even. I know you won't go wrong with either line!