Found this website that should help those wanting to do a river cruise and looking for some information comparing several of the companies;
River Cruise Reviews, Viking River Cruises vs. Uniworld River Cruises
I was disappointed that they glaringly omitted both Avalon Waterways, which in my opinion is better than Viking, and Scenic Tours, which is a very nice luxury company similar to Tauck.
So if you're thinking about doing a river cruise, which I highly recommend as they are absolutely wonderful, enjoy the information they provided, but please do not exclude both Avalon Waterways and Scenic Tours from your consideration as they are both excellent.
And as always, if you want more information or assistance with your river cruise, talk to your agent as they can be invaluable in helping you pick the best one for you.