Ok - comparison between Avalon & Uniworld - I'll try to give some comments here (more on the boats, cabins, layout, and food than on people, itinerary, entertainment since they were both charters and not the "typical" cruise). Another thing to keep in mind is that our Avalon cruise was back 2005 and Uniworld was this July - so things could have changed (or my memory fails me).
Cabin - we were in a "french balcony" cabin on deck 3 on both so they were comparable. Uniworld cabin seemed a little bit better appointed (with a small flat panel TV mounted on the mirror) and felt a little bit better. The bathroom on Uniworld was considerably better - both being showers but the Avalon one barely had room to turn around in. I would give a slight edge to Uniworld.
Boat and Layout of the boat - I got annoyed at all the 1/2 levels on Uniworld (front of the boat with the dining room & lounge is 2 levels while the rear section with the cabins is 3 levels). I don't remember this on Avalon but do remember that we had to walk "outside" to get to the cabins on the 3rd level. However, the Uniworld boat had a small elevator (something that I don't remember from the Avalon one). The lounge is basically the same with different decor. We liked the dining room on Avalon better but there are many factors that could be interpreted the other way (Avalon was all tables of 6 - while on Uniworld they ranged from 4 to 10). Uniworld's decor was "fussier" and felt less spacious (but that could be a function of the itinerary (since we were headed north - one side blinds were drawn during dinner - compared to the eastbound itinerary on Avalon which allowed for no blinds during dinner. Small advantage to Avalon.
Food - We liked the food on Avalon much better. Menu choices on Uniworld was strange - entree at the "welcome dinner" was veal with no alternatives (strange choice since many people don't eat red meat and even amongst those many won't eat veal) - vegetarian choices were usually only the vegetables from the main entree. Quality was quite a bit lower on Uniworld also. The afternoon "snack" was very skimpy, if you are not there within 5 minutes of them being put out, there wouldn't be anything left. We have never felt so consistently hungry on a cruise/vacation before. Big advantage to Avalon.
All other things being equal, I would rate Avalon higher than Uniworld. However, we don't think we would take a river cruise without being part of group (like here on LCT) or one of the charters that we've taken. We've been debating on next year's trip (whether we want to do an Amadeus Charter from Paris to Nuremberg or the PARIS one - it would be great to see many of the people from here again but its so cold there in Dec).
It feels like we've abandoned Regent but we've done all of Europe on Regent (except for 007 & FJORDS equivalents). Maybe they'll come up with some "new" itineraries soon.
