I can help,,have sailed both lines and gone to both
I ve been on Avalon 2 times and enjoyed the,,,would rate good,,,was on AMA in June from Prague to Budapest on the new ship AMAcerto,,,far superior..perhaps new ship,new staff,,not sure.
Both cities deserve some extra days,,if you can swing it...perhaps 1 minimum pre and post cruise?
Age range varies,,mostly towards 50 to 70 or so,,with a few younger and some older..but will fit in nicely.
No problem with seasickness,,,barely notice it moving...but stick with mid or top deck to prevent hearing any engine sounds..also mid ship so no anchor,thruster sounds etc.Do go for french balcony,,worth every penny...we had 2 balconies on AMAcerto and it was GREAT.The rooms are small and large window and open air helps.
Do google ea.port to find out what you want to see,,,they incl.tours if you want or go on your own(easy since usually park right near city center).
Any specific questions,,I can help(have been on 9 river cruises,,so far).