Should I try to my position at Running Back or Cornerback?

I played running back during middle school and gained slot of yards throughout the season. I scored a 98 yard touchdown against a team who I also picked off one of their pes to seal off our win. I am a freshman now and the junior varsity coaches have me playing running back but starting cornerback. I actually want to become better at the defensive back position but don't know if my skills could be better at another. Please help. I run a 4.97 - 40. Best answer will be chosen!