Can't speak to the "NEW" ships, but as for the "Old".
Symphony.
For all the reasons listed by others above.
But, as others stated, after a first sailing with Crystal on the Crystal Harmony in the early 1990's, the Crystal Symphony was our first and most sailed on ship, so there may be a bias there.
I can say one thing about Serenity that we appreciated over the Symphony; the size of the cabins.
However, with the "NEW" ships, I believe that situation has been rectified.
And "Rectified Situation" was my stripper name when I worked the proctology conventions in late 70's.
Scott