Your cruise sounds like fun and interesting. We took a similar cruise in September 1985 (a long time ago and a lot younger!
!). We were on Carnival's Tropicale (which was somewhat new then). Our itinerary was Puerto Vallarta, Mazatlan, and Cabo San Lucas. I can tell you that Puerto Vallarta is beautiful, as is Mazatlan. We toured somewhere where a film was made with Liz Taylor in Puerto Vallarta (sorry, my memory is going - maybe it was first with Richard). We have some wonderful photos of the cliff divers (in Mazatlan) - everyone should see this; it's incredible to watch. Our other photos are amazing too. It is a beautiful area. We did a bus tour in Mazatlan and they gave us these margaritas and Mexican beer. The margaritas were fabulous but strong! I was feeling happy.
We never did make it to Cabo San Lucas but did get to see it from the ship. First, on Thursday while leaving Mazatlan, there was a earthquake in Mexico City. The ship was 150 miles from the epicenter of the quake. It was a rather rocky ride to say the least, not to mention our cabin was the second from the front (it was an upgrade - yeah right). I can give you more details if you are interested. Then on Friday when we were supposed to tender to Cabo San Lucas, we sailed right into Hurricane Terry! We took some amazing pictures of the hurricane clouds over Cabo San Lucas from the ship, but it was too dangerous to tender us in.
The ship tried to make it as fun as possible. We had a costume party and our tablemates (and next door neighbors - they had the very front cabin, lucky them) dressed up as "Unicaines." Because of the captain's accident, "hurricane" sounded like "uricaine." Dan and Molly (they were on their honeymoon) decided it was like a "unicaine" and dressed up accordingly. We have pictures. It was fun. Always wondered what happened to Dan and Molly. They were great. The captain also gave us, as a memento, a map showing where we were in relation to the hurricane and earthquake.
Anyway, the moral of this story is "don't go on this cruise during hurricane season." I'm not sure the earthquake can be predicted, but we could probably have avoided the hurricane. Other than that, I think your cruise sounds great. Remember that The Pacific waters are colder too. I think I would just research when the best time is to go. If you have any other questions, let me know. I'd be happy to help in anyway I can. You also might check out the "destinations" folder on this site. Maybe some others who have been there more recently or more often than I have might have more info for you in terms of excursions, timing, and the Princess (since we didn't cruise that line).
Anyway, happy cruising!!! Let us know what you decide.