Having done the old locks and the new locks, I will say I was severely disappointed with the new locks. There is nothing much there in my opinion, so the real interest is in the journey and not the canal/locks. YMMV on that.
As for the ports, I would say only Cartagena seemed to offer a lot of interest by way of history or non-beach interest to us. That all being said, this is our favorite cruise, because we enjoy being on the ship and time at sea. We are actually doing this cruise next October as well on the Magic.
If you can get on a ship that still does the old locks and you're interested in the history, that trip might be worth it. Someone else might chime in with some great shore excursions that we missed and sway you to stay.