We took part in the 3 hours Street Art Tour with Marnie, she's a fantastic guide! She loves art and her job and she is able to transmit it to the visitors, and she knows how to involve people in the tour.
It is a very interesting tour if you like street art and if you want to explore Berlin graffiti and street art, which is incredible. Also, it allows you to discover art that maybe you don't know that exists and you can admire the real Berlin street art with all its explanations. In fact, I would have never seen all that the guide showed us, if I were alone.
For the tour, you need the AB U-Bahn ticket, but if you don't have time to buy it before the visit, don't worry, you have time to buy it at the moment, but obviously it's faster if you have it. You can buy it easily in the machines of the underground train station. We used it only once, in the final part of the tour.
If you're worried about the "3 hours tour" don't worry, it's not boring or tiring if you like Berlin street art!
I really recommend it!!