I'm with Sydneyberlin,
Epic is my fav! I think your decision has to come down to what things are important to you on a ship. I'm hearing the completely new layout of Prima is all about small cozy spaces. This seems to be causing problems with people having to wait online for an hour+ to get into some of the shows. I certainly do not consider that a good way to spend my time on vacation.
I love the solo cabins & lounge on Epic and the wide open spaces. They also have the best thermal spa on any ship I have ever been on, including other lines. Think about what things you enjoy most about the ship, and use that to inform your decision. Have fun!