Being a PhD student myself, I do not think you should ask a doctoral student.
Nor should you ask a professor who cannot comment on you in a more in depth manner.
I have been asked to write LOR for students who I have mentored before and my advice is really try to find someone who already has a degree and not only that, but a position as a professor. If they cannot cannot cannot find someone else, I'll write it (had to once) but believe me, there is very little weight to the letter when I sign it "Gella Bella, PhD Candidate"
PhD students are just that...still students. And while yes, I can definitely tell if my students are paying attention, if they are grasping the concepts, if they are interested in really learning or if they are just there to get the recommendation, a letter coming from me will not be held with the same weight as a letter coming from a professor. Its just the way it is.
Having a letter from a professor (mostly) ensures that there is a professional relationship between you two whereas a letter from a PhD student could be your boyfriend, your sisters boyfriend etc.
I personally think a letter from a PhD student is basically a waste of a LOR because it won't be evaluated the same and in my opinion will be just kind of read but not taken as seriously as one from a prof.
So I recommend trying to ask someone else. If you can talk about your research at an interview that is great in terms of showing you actually know your stuff and didn't just coast through a lab. Think about your advisor at school, a faculty member on a club or activity you're involved in(tri-beta?) etc. They may be able to write you a more personable letter than your PI, or who knows, maybe your PI will be willing to write the LOR but will ask for input from the students before he writes it. But definitely go for PhD/MD/DVM etc before PhD candidate.
Just my $0.02