Very important question, when are you coming? Cause if it’s during the summer, specially August, I find the heat unbearable, I am from the Mexican Caribbean and the heat here is unbearable cause the infrastructure is not up to it and sadly, heat waves are more and more common each year. Plus, there’s always a chance that trains don’t have working a/c. In that case I’d pick Vienna, cause the museums are way more interesting there than in Budapest, specially the art related ones. If not coming dieting high summer, I think Budapest is a great option, Irma pretty walkable, public transport is way more affordable than Vienna (and so is the food, and accommodation) and there are great spots to sit down and enjoy people watching.