We recently did the Seine river cruise on the smaller boat the Sapphire. We have booked the Rhine cruise on the bigger boat Grace. We very much enjoyed the smaller boat, and wondering if there is much of a difference in experience on the bigger one.
The biggest difference is the Loft cabins on the lower decks of the bigger ships which are a very popular, affordable stateroom option. Also the bigger ships have salons not just a masseuse (meh). Otherwise no real difference since all the older/smaller ships have gone through their remodels in the past few years. Now that all the ships have an Arthur's bistro with it's own kitchen and bigger menu and larger cabins on the Ruby Deck. The main lounge and dining room are the same. There might be more people on your cruise but maybe not. Always hard to predict.
Have said all there, there are minor differences between all the ships and from season to season. Each crew runs things a tiny bit different like where they post the daily menu, whether they put towel animals on your bed, chefs are different, etc. We've loved all 5 Tauck ships we've been on.
I know this has been answered, but there is one more difference between the smaller jewel class boats (Emerald, Sapphire) and the larger boats. I've been on the Emerald twice and the Joy and Inspire once each. The elevators on the larger boats go to all cabin decks, and on the smaller jewel class boats, the elevator only goes between the second and third decks. With my bad knees and booking the single category cabin on the lowest deck, this made a big difference in my last two trips. It was difficult on my French Escapade trip in 2016 to walk up and down the stairs to my deck, but I could do it Last December, I used the elevator all the time even to go one deck on the Joy because my knees were so bad that I couldn't do the stairs.
Good point I missed Travelcrazy. I knew they didn't go to the sun deck but have never actually used them. It makes it a real challenge for some when you have to get on or off the ship from the sun deck which has happened multiple times on our cruises.