Def decide which means more to you and proceed with your decision. Unless there is more to this...the friendship is more important for me. The fact you are asking makes me wonder if you have any feelings toward her? Your decision and hope it rolls the best for all involved in it.
If you like her, more than you value your friendship, tell her. The best that can happen is you get a girlfriend and your best friend is okay with it. The worst that can happen is you lose both. If you don't want to take any risks, tell her off.