GR???!!! I think our class is not that competitive since it does not have good reputation now. Research and recommendation letters instead of grades or GRE are most important. If I were you I would try both of them. In any cases, you can get an offer from Top 15-30, you can go to USA, but I have to say the situation for applying PHD is not good recent years due to the number students so UK is also a good choice.