This would be something that you should probably consult more about with a professor who's been on hiring committees, but I wouldn't make this decision purely based on your chosen subfield. Street art is a viable option of study in an art history program, so long as you choose a program with faculty who have expressed similar research interests. What kind of street art are you interested in? Why are you interested specifically in this subject? What about it draws your attention? If you were going to write a 100 page paper about street art, what would it look like? Which sources would you want to cite? Answering those questions could help you better decide which discipline is better suited to your career path. I just finished my art history masters and will probably choose a visual studies phd over art history, but that's due to my practice having an interdisciplinary focus.
If your interest in street art skews towards things like activism, social justice concerns, queer theory, etc etc, then visual studies might be for you. But you could still study street art in a strict art history program, and you could get a great art historical background in a visual studies program. Really it's based on differences in schools of thought and approaches to a subject. If you're interested in teaching art history, however, know that a visual studies phd will narrow your prospects at some universities.
I would seek advice from as many professors as possible and make contact with universities you might be interested in attending. Another thing you should do is look at the curriculum and mission statements at different programs - a visual studies program will often have similar courses, but you'll be able to see in the descriptions how the focuses differ across various programs.