How do I decide a field of study for grad school and whether to do a Masters vs a PhD?
In the US, most people who aspire to a doctorate apply to doctoral programs at the end of the bachelors degree. A masters along the way isn't necessary and might even cause a delay (though not necessarily). On the other hand, many doctoral students earn a masters during doctoral study, sometimes just by asking for it and filling out a request. Other times a thesis might be required. But you don't need to apply again for doctoral studies.
As to your studies, I'd suggest that you look for universities that have actual computational biology programs, or at least a few professors who specialize in that. Taking a pure CS or pure biology program might take you afield of your goal.
And no, a doctoral program isn't (appreciably) harder to get in to. In some ways it is a bit easier, since doctoral programs often come with a TA position (and a stipend and tuition waiver) and universities often need those TAs to help in the undergraduate program. TAs are not always open to masters students, nor is tuition waiver usually granted.
But apply to several programs, not just a few. And not just the &quot;top 10&quot; programs in the country. The broader your search the more likely you will find something suitable.