I'm sure that if you only have 2 or 3 days to do all of them you could. I spent a full two days exploring the castle district alone one year and when I went back the next year, I saw things that I had missed the prior year. However, I don't think that it's fair to say that either one of them (Vienna or Budapest) is better than the other one. They are totally different and offer different things to different travelers. A lot of it is dependent on what you are looking for and what you personally enjoy in your travel.... I personally like the old and drab and full of history and really don't don't get excited over the grandeur, famous music halls or exquisite restaurants... Vienna was OK, but Budapest is my preferred choice...