We did the Wonder in March and Harmony in May (will be on Allure in Sept) and have done Oasis in the past so happy to answer any speifics.
There is very much that is similar but as mentioned there are a few differences. Since Harmony has been AMP'ed it has the all the slides (Abyss and Perfect storm), along with the typical Flo-rider, skating rink and climbing walls. The enclosed Solarium on Wonder is a nice difference, but that area is 16 and over.
With the differences for the teens being pretty minimal, itinerary and other details would be my deciding factor.