Both are extremely hot, buuuut... simply based on this picture, I like the blonde's face and probably the brunette's body because of her breasts.
However, the brunette has a bit of an advantage because her pose is that much more flattering. She's sitting forward, chest out, whereas her sister is doing exactly the opposite.
If they were in exactly the same pose, I think I'd go for the blonde. Her breasts would look fuller, she's tanned, and her face looks livelier and healthier.