My recommendation would be the Regal Princess. We've sailed Regal three times and really love that ship, one of my three favorite ships. Regal is elegant with beautiful public spaces, terrific friendly crew, great entertainment and lots of great outdoor space to relax and watch the waves. The food is also very good and the British vibe is alive and well with fish & chips and bangers and mash available at the Wheelhouse Bar on sea days.
The Piazza is the ships atrium and main gathering place where there is always some fun event going on from line dancing to champagne waterfall on formal night. Princess is a great cruise line and Regal truly celebrates the classic feel of cruising on a ship with lovely lounges like Crooner's, a perfect place to sip a cocktail, listen to piano music and either gaze at the sea or people watch.
Remix is the name of Regal's teen club where they'll find foosball, skeeball, even a DJ booth where they mix their own tracks. At the sports court upstairs they'll be busy with basketball, ping-pong, the swimming pool and special organized teen events.
We've not sailed NCL Star. As LeaveitallBehind mentioned she is older, built in 2000 but, refurbished recently in 2018. Typically, I would always opt for the longer cruise. But, my pick is for Regal don't miss a chance to cruise this gorgeous ship next summer.