I think it's up to y'all...we have done 2 holiday cruises (Danube and Rhine) and I went in almost dreading the Danube because I just wasn't interested in it (we went because that year-not done in '17 nor scheduled for '18) the Vienna Boys Choir for Christmas Mass was part of the trip and DH had to go!
That said, we loved our trip. We took the Rhine Holiday this year because we enjoyed the format and it would give DD "built in friends."
I view the river cruises as expensive, European all inclusive resorts- you pay for food, a room and some entertainment (there isn't a whole lot of options-usually a performance or something in the late afternoon/early evening but no club/casino, etc. There is a bar though!) plus excursions. Wine flows freely at meals (if you're a wine drinker) and the food is good (note-not a foodie and often ate from the kids' menu...but I didn't hear complaints from those who have more sophisticated palates). The boats are much smaller but the common areas are well designed and the staff very nice. Honestly, I feel they are a lot like ocean cruising except smaller and go to different places...some of which are lovely little towns. It was very nice, for example, to get off in Passau and just wander around and look for presents for a German born friend of mine. In another town I was on a quest to buy diet coke and a pretzel (it had wonderful Christmas markets but my needs were very specific that day

).
Before we went on our first river cruise, I listened to a
Dreams Unlimited podcast by John and Kevin about their Seine River cruise with Viking (I believe) and read every single trip report (thanks
@sayhello,
@calypso726 and others). It definitely helped with expectations!
Honestly, for a honeymoon 2 days in Paris (or more) sounds nice, then relaxing on a river cruise soaking up culture and spending time with your new husband sounds lovely.
I know you've been weighing all the options so I know whatever you choose will be great.