Oh sweetie, I know how you must feel, I lived the same thing, it's so hard to choose, a child shouldn't have to do that, that's so sad and terrible !
As it seems you love both the same way, to 'choose', the best is thinking of where your friends are, where your school is. That's terrible to say that but the only factor to choose you have is the practical factor
But you must also think of who of both of them will be the most easy to live with, I mean if your mother is really, really sad, she may need some time for herself and maybe it rather would be better to be with your dad.
I talked by experience. At the beginning I 'decided' to live with my daddy but quickly I realized he was not able to take care of me and to be okay in our every-day lifes. He didn't do the housework, he didn't go to the grocery store... And worst of all, he cried all the day.
So, I went to my mum's, it was better for myself and my mind.
You must think of you. But remember, if you go toyour mum's, your father won't think you love him less, your parents know how hard it must be for you.
Don't feel guilty, what's happening is not your fault !