Janfred, Of the "Big Four" river cruise suppliers I have been on three of them. Uniworld, Avalon, and Amawaterways. Believe me, I've got my eye on Viking.
. They all pretty much offer the same amenities; included shore excursions, 24hr coffee including lattes, cappuccinos, wine, beer, soda, with lunch and dinner, free internet, some have complimentary bikes to use.
Uniworld does indeed have very elegant looking ships. Their decor is modeled after the Red Carnation hotels which is deluxe. Uniworld and Red Carnation are owned by the same parent company. There are a lot of elegant touches of padded cloth walls, marble bathrooms with heated towel racks, chrome free standing toilet tissue holder, tufted cloth headboards, floor mats by the bed that say "Good Night"/"Good morning".
I would have to say though that which ship you pick can make a difference even in the same cruise line. I went on the brand new Uniworld River Antoinette. Because of water levels on the Rhine River we had to do the first two days of the cruise on the River Princess, and then switch to the River Antoinette. The River Princess wasn't anywhere near impressive as the River Antoinette. Had I only seen the River Princess and not the Antoinette I might not have as high of praise of Uniworld. The service and food is superb though throughout.
I find Avalon and AMAwaterways very much on the same level. The amenities are almost identical, service and food very similar. So things I would compare when choosing between them are age of the ship, itinerary, and price.
It should be noted that some of the newer ships have more amenities such as indoor pool (albeit small), laundry room, al fresco dining on front of ship. It should also be noted too, that some of the newer ships also have more passenger capacity. One of the things I love about river cruising is how quickly you embark/debark because of so few passengers, and the space ratio. Some of the newer ships boast regular balconies over the French balcony. But beware, some of them are taking space out of the room for the regular balcony. The new Viking Longships have two room suites to allow one side for the regular balcony, and the other room with floor to ceiling window and still give you space.
I don't think you can go wrong with any one of the river cruises.