Which is more picturesque taking the train from Budapest to Salzburg or taking the river boat from Bratislava to Vienna than onto Salzburg?
I had thought about something similar once so I have the links to two boat companies. I haven’t done it and I haven’t spoken to anyone that has but I have spoken to a couple of individuals that did do the Vienna to Budapest hydrofoil and they both commented that it was a rather harsh ride; and the Bratislava to Vienna trip is on the same sort of boat (at least on one boat line). I don’t think you can get on deck on any of these so all the view is through large windows. I also haven’t been on a train between Budapest and Salzburg but if it connects in Vienna then I have made the Budapest to Vienna ride and its rater unremarkable. What is a nice trip is Szentendre to Budapest by boat. This is a slow speed boat where you can actually get out on the deck and watch the world go by. Done that a couple of times and enjoyed it.
If you want to take a boat, take it all the way from Budapest to Vienna. There are several parts of the journey which are quite picturesque. Unfortunately it was mostly raining when we did it. It was a hydrofoil so there was no deck on the top but there was one in the back of the boat. The ride was smooth most of the time. The ride is certainly more picturesque on the boat than on train between Budapest and Vienna. Between Bratislava and Vienna you can see castle Devin on a steep hill, otherwise the countryside is flat with a lot of deciduous woods.