We've cruised with AMA and Viking. AMA's food is much better. So is their service. They had an elevator down to the dining room. Viking only had stairs. That's not a deal breaker for most people, but if someone in your travel group has mobility issues. stairs could be a deal breaker. AMA had scooters we could reserve for excursions. With Viking, our
travel agent had to get permission from the higher ups, and threatened to pull her business if we couldn't take DH's very necessary
scooter. When it wasn't in use on excursions, we kept the scooter collapsed and in our stateroom, so there were no issues.
The time we cruised with Viking, it was because of a particular itinerary, in a shallow river and Viking could handle the depth better than others. (Their ships have flatter keels.) They're more of a discount company -- cruising with K-Mart. YMMV. They do have a loyal following.
FWIW our travel agent recommended the Christmas market cruises, but I'm a desert rat and don't handle cold weather well.