Now, I love both of them, but I think they're very different Doctors.
Tennant's Doctor didn't seem to really care when he wiped out his entire species besides himself, and was very happy go lucky all the time, which I didn't quite get, also, he was made into too much of a romantic human I think towards the end, but that's more down to writing. Playing the part in the well-written episodes he was fantastic. I loved that episode where he and Martha had to beat Richard Lazarus who was sucking the life from humans in order to stay young...
Smith is the youngest Doctor there has ever been, and I think therefore had more to learn when he began, but has now got into the role amazingly and I love him. I think the writing has improved on a whole since he joined as well, which is nothing really to do with him, because he's not the writer, but still. My favourite ever Doctor Who episode was 'When The Doctor Met Vincent', but that was probably more to do with a love of Vincent van Gogh than Matt Smith.
It's a hard one. In the end, I think I have to choose David Tennant, because he's more sexually appealing. That has just rendered my last two arguments pointless. 
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'I may not amount to much, but at least I am unique.' ~ Jean-Jacques Rousseau
'I sometimes go to my own little world, but that's okay, they know me there.' ~ Joel Hodgson