Have been on both lines,,and really difference
is very slight,,,give a thought as to the weather on xmas tours,,will be cold and even may have sleet,,etc...and best thing about river cruises are the scenic views on top deck as you float down river..
Do check out ea.ship on websites,,,I like at least a French Veranda,,,on top deck,,,new ones even have 2 balconies on ones in 2012...
Go for itinerary that best suits you,,if pressed ,,,would give slight preference for Uniworld,,,for service and food...PS..if you look at fine print on 2 for 1 offers on Viking,,the balance must be paid when booked...not 90 days before like most,,,do check out a few online TA websites for best prices..and OBC...
any other questions,,glad to help...good luck,,,