I actually haven't done a river cruise, but you really are talking about apples and oranges here. A river cruise is focussed on a very specific area versus a large ship cruise, where you visit several countries and different areas within countries. As far as structure, actually the big ship cruise is going to give you more different things to do, by virtue of how many more different types of entertainment they provide. Suggest you talk to a cruise only travel agent, who can help you compare them better.