Obviously O’donnell.
Delaware is not Alaska. O'Donnell cannot win the general election. Castle votes against Obama about half the time. That is much better than the Democrat will do. In addition, Castle can help Republicans gain control of the Senate, which (among other things) would mean Republican control of committees that can investigate the Obama administration's lawlessness.
I'm all for voting for the most fiscally conservative candidate who can win in a general election; but the key is the “can-win-in-a-general-election” part.