I would have to vote for Nintendo as I have never really tried much from Sega...... One or two Sonic games briefly, but that is all, so I vote for Nintendo.
As I said in the thread regarding this subject, Nintendo characters have much more personality than Sega characters. I have to vote for Nintendo for this reason.
Eh, more of a Mario vs Sonic battle than anything, but I'd have to say Nintendo. I actually really like the old Sonic games, but they're just not as good as what Nintendo has to offer.