I have been on 4 river cruises - 3 Viking and 1 scenic. We enjoyed all the cruises. Scenic had two special dinners, one was for people staying on the third floor which was like a Chef's table and the other is a dinner serving the local flavor in our case it was French.
Both can include air and transfers and one tour a day. Viking had some optional tours in the evening for a reasonable price. Scenic is truly all inclusive, Viking you pay for drinks outside of lunch and dinner. If you are doing Paris, Viking is not the line to use since their ships are too large to dock in Paris. Our May cruise was on Scenic and dock within walking distance of the Eiffel Tower. There were no children under 18 on either line.
The service you get is similar to the suite experience on a RCCL cruise. The linens and towel are high end and cabin service is quick and discreet. River Cruise ship Cabins are tiny compared to Ocean Cruises. Getting a balcony is nice but often times river cruise ships are rafting meaning you are inches away from another ship making the balcony unusable. Sometimes the Veranda's include perks such as free or low cost airfare so you need to figure out what is important to you.
That being said, we enjoyed Scenic, we won the cruise so we were spoiled, I do not think it would be worth double the cost compared to Viking. If I went on another Seine Cruise, I would not do Viking since they do not go to Paris. I would look at Scenic, AMA, Avalon, Vantage , Emerald ( the sister lower cost line of Scenic)all who were able to do the whole river and then choose the one with lower price.