I've done the London/Paris trip as well as the Danube River cruise. I have links to all my Trip Reports in my signature. I did a lot of the activities from the Italy ABD as an add-on to a
DCL Med cruise, but I really don't feel like I can speak to the land ABD, so I won't.
If you want your first ABD to be a true ABD experience (family-feel to the group - ie, getting to know your fellow travellers & Guides well) then I'd suggest doing London/Paris over the Danube River cruise. The River cruise was a FABULOUS trip, but I missed the family feel of the land trips.
That said, I don't think you could go wrong with either trip. London/Paris, as
@tink1970 said, is more accessible (easier to get to/from, English spoken everywhere in London, etc). However, the differences between the places you see on the River cruise is really cool. There's a wider selection of activities that you can choose from on the cruise, being really active or not as much. And tons of folks spoke English in this area of Europe, also.
Personally, I think I liked the Danube River cruise a bit better than the London/Paris trip. Just more varied, and a bit more "exotic" for me. (Although if this is your first trip to Europe, everything will feel "exotic".

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