In terms of personality, Beatrix has a little more going for her. Though I feel that Leo works a bit better since his character is more symbolic than anything, whereas Beatrix is pretty menacing until her Face Heel Turn. I kind of wish the writers had done more with both of them as I felt like Leo was a really interesting character and Beatrix could easily have had a larger role if she hadn't been written out of the plot after Alexandria gets destroyed.

Impact-wise, I felt that Leo was better because you almost think you're about to get this cool character in your party and then he's unceremoniously killed less than a minute after you control him. Considering Revive Leo rumors had existed long before Aerith took that spotlight away from him is still a testament to how cool he was.