I had the opposite reaction to Eccleston and Tennant: I always found Nine believable, while Ten was just too ridiculously over-the-top to ever be credible ("equal believability" in any mood, sure - 0=0). Especially in Season Two, Tennant overacted like crazy - though he did get better with time, and was mostly bearable by Season Four. I suspect this may have been habit due to Tennant's coming to the role from the stage, where you have to overplay everything because even the good seats are several feet away; film and TV acting requires more subtlety, since the camera can get right up in the your face with a close-up for the critical character moments. I also liked (and believed) Eccleston's on-screen chemistry with Billie Piper better than Tennant's.