I'm partial to RCI, but in this case, because you want time together and Celebrity is a tad lower key than RCI, I think I'd go Celebrity.
For a first cruise, I'd want to leave from a closer port, less hectic getting there! Also the seas days are relaxing, I wouldn't want to do a port intensive cruise for my first cruise!
On the other side of the coin though, you could relax on a beach at each island to get away from the crowds!!!
It is a tough call, Millenium doesn't have ports but has those nice sea days. San Juan is nice, old San Juan is close to the dock and fun to walk through, plus the fort!
Now I've probably added confusion!!!