I think face to face too. As a spouse of a survivor, I can say when the word abused came from my husband, it sent a shock throughout me. It also made me want to hug him quickly. Also if you plan to tell her how you've dealt with it (acting out, depression, etc.), in person would be a very good way to tell her.
It is great you are telling her now. I don't know your ages or how long you've dated, but your getting help now instead of later in life is great. You and she both will be able to handle things much better then having more years in your story.