I have no basis for making a statement based upon my own experience on those. My time aboard ship was on a Royal Caribbean ship, and most of the “big” lines all do almost exactly the same thing as each other. Centered around July, they do Baltic cruises which are Scandinavia, Russia, Estonia, typically starting and ending in Britain, but they can start & end anywhere. If they go to more southern Europe, the Mediterranean, they have a few more months of operation, maybe even all year. If the rivers you are going to peruse are in northern Europe, think July + or - a month. If southern Europe, those might go all year around.
The smaller cruise lines are quieter, smaller vessels. Some are very high end, probably examining the competitive pricing will tell you. The Royal Carib boats have roughly a 3:1 pax to crew ratio. As one data point, the Jewel of the Seas, had 2200-2400 pax and 850 crew on board. Some of the smaller boats have under 1000 pax. The smaller boats are going to have fewer rock walls and bowling alleys and that kind of silliness. They will all have many bars and casinos, that much you can count on.
I don’t think you can go too far wrong with almost any of these. If they suck, they get talked about and people would flee. If they portray themselves as top drawer and really cost a lot more than the competition and are only mediocre and fail to meet gourmet standards, people will bolt. It is a very, very competitive industry and there are very few lines that have any particular handle on any one destination. So if one makes folks unhappy, they will suffer. They all seem to try to do a good job, all the workers are watched carefully and rated and the lines really try to keep things under control, quality wise. Sometimes they slip up. When that happens, they know passengers will start demanding refunds and offsets, etc; and the cruise lines HATE to give away money. The recent Italian episode was apparently one drunk captain trying to impress a gal he was working on. Yes, pretty danged bad, but not typical of the industry as a whole.
Just try to go with a boat built within the last say ten years and a reputable cruise line.
Wine and beer with lunch and dinner are included, and you are allowed to purchase wine and bring it on board. The cruise itself is pretty pricey: almost $9K for two people. (That's the part that's holding us up ;-)
My husband and I are taking the Disney Wonder to the Mexican Riviera next November. We had discussed staying on the ship, and I think this convinced me.