So the difference between Nine and Ten

[misaffection]

is that Ten makes you want to ruffle his hair, whilst Nine makes you want to jump him? *laughs*

Actually I think, in a nutshell, that's how RTD is writing this. I mean, Chris is all brooding and mournfully handsome - perfect for an agst-ridden Doctor just escaped from the Time War... whilst Tennant is cheeky and baby-faced handsome - perfect for getting away from that and injecting some humour.

I believe it was John Barrowman who said of the Doctor/Rose kiss that once he'd done that, he could draw a line under the whole thing. It was the last act of his love - to save her, then let it all go. The arc is complete, time to move on.

In every sense of the phrase.