Face to face is good. But you can be very smooth/affectionate by writing it in a letter, and tell her to read it when she's alone. Once she reads it, face to face will automatically follow afterwards. Gives her time to take in your feelings and contemplate hers, not to mention gives you enough time to contemplate what to say to her, no matter her response.
I think it depends on the person you're dealing with. Some people need to hear it in their face, while others can't handle that. Some don't even want to hear it. Heck, I know people who don't even care.