2015 will be the year of dwarf planets
And both attractions will start more or less in same time - Dawn should enter Ceres orbit in february, same month the New Horizons will begin observations of Pluto. But it won't be untill may when quality of it's images will exceed that of Hubble. Flyby will take place in mid july
I can't make up my mind between those two. But I guess there is more thrill involved in New Horizons. Dawn already is a great successs, giving us huge ammount of data on Vesta. It also moves slower. New Horizons on other hand is hurtling towards Pluto and it's unknown number of moons, it has quite limited time to get all the data, for all we know it may just crash into some icy body in Kuper Belt.
"I walk alone and do no evil, having only a few wishes, just like an elephant in the forest."
"Laugh, and the world laughs with you. Weep, and you weep alone."