I like the sunset bar better on Eclipse, along the wall instead of an island.
I like having the Lawn Club Grill on the Reflection rather than the hot glass show.
My favorite room on S class is on the pool deck, so doesn’t exist on Reflection since there are no cabins on the pool deck.
I think the Sky lounge is bigger on Eclipse because it lacks some suites that are on that deck of the Reflection.
Reflection is the most different of all the S class. It has a higher passenger capacity, some find it feels more crowded but I haven’t noticed that.
If you want to try something slightly different then pick Eclipse.