I'm in the minority that if I could only have one previous Doctor on SD, it'd be Eccleston because I don't think that Billie and Tennant have "hot" chemistry. They have sweet, bubbly, bouncy, joyful lovey-dovey chemistry, but it's never struck me as hot. Unlike she and Eccleston.

Not to say I'd be upset by Tennant on SD, I'd just prefer Eccleston. Actually, I'd prefer my long-dreamed for scenario where Eccleston and Tennant BOTH guest-star as Doctors/boyfriends who hire Belle for a threesome for a birthday present. And, of course, because they want it anonymous, they make her call them both "Doctor." That's what I REALLY want.