I would suggest heading to Cruisecritic.com and find a thread for the specific company that you are looking at, room selection is always a big topic, it will vary by the company, the boat, and the river you are cruising on.
Some comments I see repeatedly, first, no, you are rarely just sitting on your balcony "cruising", and when docked, you are likely to be within arms length of another docked boat, and docks are not usually in the most scenic part of town anyway.
For days when you are cruising through the most scenic stretches of river, you are better off being up on the top deck, better view, you can see both sides of the river, maybe some drinks or snacks, maybe they even have some guide/narration going on.
YouTube also has a number of "river cruise" videos, ranging from pretty good to the typical YouTube slop. Room selection is often mentioned, if I recall the general advice, it is don't pay for the most expensive rooms unless you need more space, the mid range rooms are popular, experienced "cruisers" watching their budget just get the waterline rooms, and stay away from the engine room.