We've taken both, although not in Europe.

The river cruise is more intimate, with a smaller ship and fewer passengers. You will be in sight of shore the entire time, so the scenery is more interesting. Heavy rains before or during your cruise could cause delays or cancellations, as the boats must be able to navigate under bridges on the rivers and if the water is too high they cannot. We found our river cruise (China, Beijing to Shanghai) to be more structured than a megaship cruise, with most tours scheduled and included in the price of the trip. On a megaship cruise, most shore excursions are optional and not included in the cost. Your river boat will have one restaurant, with no room service and no alternate venues for meals.

Larger ships are more crowded, but there are more on-board activities. They have more than one restaurant, offer room service, and have plenty of dining options. We preferred the river cruise to our megaship experiences because of the lack of crowds, but people we met on the ship felt just the opposite because of the lack of a sizeable onboard gym and spa, and shows and other entertainment venues like a casino. It would have been nice to have more than one dining option.

It's all about what you like.