I'm entering a PhD program with two options -- pure math and mathematical computer science (MCS) -- and I'm torn between the decision. As an undergrad, I loved pure math and that's why I applied to grad school. I have no doubt that I could stay awake at night thinking about differential geometry for the next few years and really enjoy it. My experience with MCS is limited to a couple chapters of an algorithms book and a few weeks of Stanford's machine learning MOOC. What I've seen has been interesting and beautiful, but I'm not as confident I would love it. As choosing a track requires planning courses semesters in advance, I cannot easily switch after choosing, so I want to make the best decision from my current point.
So, my question is: How can I tell whether or not I would enjoy mathematical computer science? How does it compare in flavor, style, to areas like analysis, algebra, etc? How might doing a PhD in either area differ as a student, and how might it affect future job prospects? Is it a good idea at all to make a switch without in depth prior knowledge?
I am not looking for answers that advise me to question why I am or if I should be in a PhD program since I'm undecided, nor am I looking for answers that are basically 'don't worry about it, just find something you enjoy.'
I realize this might not be the right forum to post this in, but I didn't think it fit the academia stack exchange. Please let me know if there's a more appropriate place to post this.