Without hesitation I would go n the Millie. I think you will enjoy the ship for starters and it is a couple of more days at sea.
It won't have the bells and whistles that the S class has and not as new of course but you can't go wrong.
I would rally like to try the Connie as it has be "Solstized"
Well my wife won the debate about this one. We booked on the Millenium for October. What's funny is now that we've finally made the decision (M vs. S) I'm so relieved and looking forward to sailing her.