we (DH, DS8, and DS4, and me) did a 7-night alaskan cruise to alaska last summer, then 5-night stay at aulani this past april. there are links to both trip logs in my signature below.
whenever anyone asks me this question i would first ask is if their child is 3 or above (which your younger one is) because the kids' clubs are 3-12, and you would have to pay for the nursery (ship) / babysitter (aulani) by the hour if they're 0-2.x. but in your case you don't need to worry about it.
second question is whether your kids still nap. i ask because my younger one was still napping when we went to alaska, which cut into our excursion time a bit, so we only did half-day excursions. if we had to do it again we would go to aulani first (so she could nap in the hotel room), and wait to take the cruise until she no longer naps. after she dropped her naps in the winter, it totally opened up our days when we went to aulani in april--we could just play all day long and not have to watch the time.
honestly we really enjoyed both trips equally! they're very different. there was so much to do on the ship all day long (excursions during the day, evening shows at night, for example; there are many many more activities throughout). when we wanted some adults time we would just drop off both in the kids club. the meals are "free" because they're included as part of the cruise costs. but the dinner times are set.
aulani is much more relaxing overall because we just spent the whole day in the lazy river, pools, and beach, and there were no excursion nor meal times to meet (other than reservations at ama ama). BUT their kids' club process is more ornerous, in that you have to pre-register your kids, and you can't just drop them off whenever you want to because of capacity issues. with some pre-planning you can do it, but it's just not as easy as on the ship. and the meals are relatively more expensive.
so there are some things for you to consider with both destinations. but either way you will have a great time!