Kickboxing or Brazilian Jujitsu?

Kickboxing. It takes years to master BJJ well enough to subdue a much larger opponent. In BJJ a twenty pound weight difference is huge, just ask anyone who competes in the absolute division in a grappling tourament. It would be stupid for you, a small woman, to pull guard or shoot for a takedown where you can be easily out muscled. If you learn kickboxing no matter how big an opponent is you can back peddle, cut the angles to pick your shots, and get on the inside of their longer reach and do damage. Just make sure you're training by hitting mits and sparring with live opponents and not doing some ridiculous cardio kickboxing class.