What can't you start both? I can't imagine a pitching staff so stacked that there's no space for both of them in your lineup, especially with 2 2-start weeks.
I have to agree with Schmidty, why not both?? I know I would have them both in there myself. It's pretty much a wash between the two of them anyway, although if you need more K's I would go with Lincecum since Halladay is more of a ground ball pitcher.