Peter, we did the Panama Canal trip back in November. It was a ten day trip that left from Puerto Rico and stopped in Aruba (been there, done that), but then included stops in Panama, Honduras, Costa Rica, and ended in Acapulco. I always wanted to stay at Las Brisas in Alcapulco (remember the movie "10")and see the cliff divers so we stayed five days there making it a two week trip.
To me, a Panama Canal cruise is all about the canal and the other stops are incidental. The passage takes nearly a full day and seeing the different parts of the canal (from the beautiful lakes to the dirt ditch), the operation of the locks, and the watershed that supports the canal made the trip for us.
Personally, I would not take a cruise all the way to San Diego (or LA)as those are areas that I have visited many times before. It just really depends on how much time you have - or want to spend.
Maybe someone wiser than me knows how you could get a better introduction to the Canal on a 14 day cruise as opposed to a 10 day cruise, but I can't see how. Check out how long you will be in port and what kind of tours are available. Our ship had tours to Panama City and two tours that concentrated on visits to the locks and a review of their operation.
We just completed a Transatantic cruise and I was a little concerned about getting bored spending five consecutive days at sea, but it was actually extremely relaxing and fulfilling (did I mention the green bikini?).
The point being, if you have been to a port already, stay on the ship and relax - you don't have to go ashore. You see a lot of posts that say the Ship is the destination - not the port. While I don't fully subscribe to that approach, there is some merit to it. Take some good books, sun tan lotion and chill until you come to a port you would like to visit.
But if you REALLY want to get an introduction to the canal, you can fly to Panama City and then swim the canal. The record time for the swim (as far as I know) was set in 1962 when Albert Oshiver swam it in 29 hours. He had to pay tolls based on the standard for ships and, for him, it came to 45 cents.
I am sure the rates have increased since then, but it would beat the price of a cruise and you would be one with canal - can you say oooommmm