Your TA has recommended the least crowded ships (Tauck), the most crowded (Viking), and the most polarizing decor (Uniworld). I would also suggest AMA and Scenic, to round out the luxury category.
Pay attention to cabin size and layout, number of passengers (almost all ships are the same size, based on lock size - and they go from 130 to 190 pax, a huge difference!). The other important factor is itinerary - at first blush they may all look the same, but the details vary in ways that may be important to you. I especially like Scenic’s 10-night itineraries, which add so much detail on the same stretch of River.
As to price, be sure to take into account the different inclusions.