choochella, as always, has given excellent advice.
I love all the Princess ships--big, small, new, old--so I usually pick my destination and then see which one(s) are going there.
The Grand Class ships have the advantage of more dining choices and entertainment venues; and also mini-suites. Yes, I know the Sun Class ships have mini-suites--but they are much larger and more expensive than the Grand Class ones.
The smaller ships are, well, smaller! Easier to get around, more intimate, and maybe even more "comfortable"; more of a "ship feel" to them; less like a floating resort.
As for "Movies Under the Stars", I have not experienced this yet, but it does not appeal to me AT ALL! I LOVE movies, and I can think of no worse place to watch one than the open deck of a cruise ship; but that's just me.
Anyway, hope this helps. Let us know what you decide.