My suggestion though is to just go with the PhD. You don't know what's going to happen in 3 years and I honestly don't think an RAship will cause you to jump from a top 20 school to MIT or Harvard. If that's the case, then the difference between your school and a school that you may get into will probably be small. Why add risk to something when the reward may not be too high? It's a different story if you got into a top 20 school that you don't really want to go to though.