Stan,
There is no comparison. We have taken 3 Viking River Cruises and 6 Tauck River Cruises. While Viking is considered 3 star, Tauck is 5 star. Now, that said, you are in Europe, so it is a great trip either way - but once you go with Tauck, it's hard to go back to Viking. Now, we have gone back to Viking for the right itinerary, but we go knowing what the differences will be.
Tauck is truly all inclusive-- including unlimited top-shelf wine, beer, cocktails, all tips are prepaid, all port charges are paid, and the excisions and tour guides in the towns are the BEST. You will learn and see so much more than with Viking, Avalon, Gate 1, AMA, etc. And there are lots of great surprises and treats along the way.
The 'so called' included Viking excursions in the towns are not the same level as the excursions you will get with Tauck.
Sometimes with Viking, the excursions are simply a walk thru town, pointing out a few things, and handing you a map.
With Tauck, you are divided in small groups with a local guide and you spend a couple hours learning about the area, entering castles and cathedrals, hearing amazing organ concerts, sampling the local cuisine, etc - then there is ample time to explore on your own before going back to the boat.
The food is also better with Tauck with amazing regional cuisine, other choices if you like more traditional, and another restaurant onboard that has fresh pastas, sandwiches, burgers, soups, etc.
They also bring local singers and dancers onboard for nighttime shows and you'll find a 'local treat' in your stateroom many nights.
When comparing Viking to Tauck, it's really hard because you can't really compare because they offer such different experiences. The airport pick up alone is VERY different and the PRE and POST hotels are VERY different.
All that said, there is a new 12 day Xmas Market Cruise that Viking just posted that I am looking into because I like the itinerary.
We actually like Uniworld and AMA better than Viking. We have also done GATE 1, which was just OK for us - again, you are in Europe seeing amazing towns, but the actual experience and SERVICE and all-inclusiveness is VERY different.
By the time you add in the tips, port charges, alcohol, airport transfers, TRUE excursions, the price isn't that different.
Our friend is a Travel Agent at AAA and she books for all the lines and she cruises them all = she has done a spread sheet for us and the price wasn't that different. She has many complaints from her customers with Viking and the flights that Viking books. She had many issues with Uniworld for COVID refunds. Again, she cruises them all and books them all, but Tauck is also her first choice when people ask her.
I think if Viking is more what you are looking for, or Gate 1, or whatever - GO FOR IT. The rivers in Europe are amazing and it's a great way to unpack once and see several parts of Europe with a guide and nothing to worry about.
I certainly wouldn't go into debt to go with Tauck if you can't afford it.
I do agree with who posted above - I like the LAND tours the most, but the River Cruises are easy and a great way to see a lot and not have to unpack every few days. Again, the Danube Xmas Market River Cruise is a GREAT way to start - but they book and fill up a year in advance.... esp if you like the cheaper rooms like we do.