I do not think that there is a lot of difference in the passenger experience generally between Holland and Celebrity. The bigger difference is between ships, time of year, length of cruise, etc. Like RonDon, we often enjoy the smaller ships more - we really enjoyed the Maasdam on a 2018 cruise, for example, in part because of the significantly smaller size - about 1200 passengers to the well over 2000 on Summit. We cruised Summit in 2008 and enjoyed the ship very much, but all other things being roughly equal, we would choose a Massadam size ship (or other smaller Holland ships) over Summit size. Holland has the reputation of having older passengers, but I do not think you will feel any difference between Holland and Celebrity on that measure, in part because you are on a specific theme cruise which probably has lots of repeaters and in part because I think the two lines have become more similar in the demographics.