I think, between the two of them, definitely Mao Asada, because she's already proven that she can compete with the Senior ladies by winning the 05 GPF, and if anyone has seen her 2006 Japan Open program, her body's growth gives her extra strength to land that triple-axel rather than plain deadweight.
Fact is, we've never seen Yu-Na Kim compete with the Seniors as Mao Asada has. Some people's success in the Juniors does not translate elsewhere. Miki Ando, Jenny Kirk, Andrei Griazev-- all Junior World champions, but did not have a 'breakout' year. Mao Asada has already had a 'breakout' year and she hasn't even gotten out of the Juniors.
I guess only time will tell. Personally, I'm just happy that we have such talented skaters coming in and keeping me interested.