I would choose by itinerary, not by ship, if possible. Do you prefer a one way or round trip? Vancouver or Seattle departure?
You have 3 ship options on RCI:
Serenade of the Seas has one way itineraries departing from Vancouver or Seward.
Radiance of the Seas has round trip itineraries from Vancouver.
Ovation of the Seas has round trip itineraries from Seattle.
One way itineraries let you see more of Alaska and add on a land tour in Alaska.
Round trip itineraries let you depart and return from one port/airport, and you can choose between departing from Seattle or Vancouver, and possibly adding land time in one of those cities.
Vancouver departures/returns will be more scenic because you are sailing in the Inside Passage vs the open ocean, which is where Seattle departures sail. Inside Passage is smoother, whereas ocean is rougher.
You won't have many sea days on any itinerary, but when you do, will you want to be admiring the scenery and looking for wildlife (we spotted whales, dolphins, otters, and sea lions on our cruise) or will you want to be inside doing activities?
The sailings all have slightly different ports. Do you want more Alaska ports or Vancouver/Canada ports? What about glacier time?
Also consider that RCI will be allowed in Glacier Bay, which IMO is a must do, and it will likely be a smaller ship that is sailing into the bay.
We sailed on a roundtrip from Vancouver on another cruise line (HAL) which my teens LOVED, and which visited Glacier Bay. We spent 4 days in Vancouver/Whistler before we sailed. If we go back to Alaska, we will do a one way with land time in Alaska.
This is Glacier Bay. These are not photoshopped. These are real photos that I took with my camera or phone.
You can't really go wrong with an Alaskan cruise, but you really should prioritize by itinerary/departure/return, then if applicable, choose the ship. FWIW if we had sailed on a ship with a million activities it wouldn't have mattered because my kids spent all of their free time in their respective kids clubs with their friends. My son and his cruise friend wandered a bit, and joined me in the lounge to watch for wildlife a couple times, but they would not have cared about a rock wall, sky diving, or whatever is on the bigger ships. DD would not have done any activities if there had been any. She was 100% fine chatting with friends all day and night.
Also, any itinerary in Alaska is VERY port heavy. You will be out on excursions all day, and you will return to the ship feeling exhausted, only to wake up and do it all again the next day. You probably won't have much time or energy for activities.
You know yourself best, though. Think through all of the options. Happy to talk more about Alaska any time!