Yep, this is a tough one. I do need to consider my Mom's preferences as well, and while I might lean towards the PG and the more active vacation, I have to agree with Donna that my Mom would probably enjoy the Mariner more. She's definitely NOT a water-sports type person!
I'm reluctant to miss the motu's, tho -- so many people have written about them as if they are the most idyllic day in paradise. Mom says she'll enjoy whatever cruise we do...we already agreed that she would go on some different excursions than me, just like we did on our Med cruise.
But in terms of actual time on islands, it appears that I would get more time on the Mariner cruise. It visits a total of seven islands, with 8 to 10 hours on each (except for Tahiti, where it stays for 2 days.) I'll have to sit down and actually compare hour-for-hour how much time we spend on islands on each cruise.
I'll copy David and quote Jack Benny again: "I'm thinking..."
(Oh, and what an exciting dilemma this is, tho!)