I've not travelled with the lines you mention, so can't help there.
We spent little time in the cabin, though the french balcony was nice to have. Mostly did sightseeing from the sun deck, which unlike the big cruise ships is only a short walk from the cabin. Personally, I would choose more room in the cabin itself, or lower price.
A lot of the entertainment/ sightseeing is away from the boat - a good pair of walking shoes is recommended (though there are different options for older/ less agile passengers).
One thing to be aware of is that weather, leading to high or low water levels, can affect the itinerary (too low can't travel, too high can't get under bridges).
Lastly, there's a section on Cruise Critic (UK site) that may be useful:
http://www.cruisecritic.co.uk/river/index.cfm?sr=uk
Hope that helps!