Thats a depender.....
If you want to go from point to point and say "wow" at each point, take the van. Or if you have a disability, take the van.
If you want to see Budapest, learn a little of the culture, experience the people and maybe make a friend or two; take public transportation. Every major site in Budapest with very few exceptions is located within a few minute walk of one of three tram lines and one metro line. One of those tram lines is always described as one of the most beautiful tram rides in the world. Makkay, Shmakkey. If he/she is trying to cram you in a van, I sort of wonder.....
While i wouldn't take a van, i would be inclined to do two half day tours (in the mornings when it is cooler) so you have time to do some things on your own; and there are a lot of things to do.
Honestly, unless you have a particular interest you want to learn about in detail; Jewish Budapest for instance or are very limited on time, i would think twice about a guide. But a lot of that also has to do with where you are staying and maybe the size of the group. Not a bad thing, just not essential. Its an easy town to navigate.