I'm an agent with a travel company and can vouch that both Viking and Avalon are wonderful river cruise options. I'm saying this from my clients' feedback from numerous bookings. Viking is rated #1 in a lot of categories, and their pricing seems a tad lower (in general) than most other luxury river cruise lines. The hold times for Viking are much quicker than Avalon (in general), if that makes a difference for folks. Others to look at might be American Cruise Lines, and Uniworld, for river cruising.