I would start Ortiz, because he never really tends to get rocked even on his bad starts and even against tough offenses, so his chances of destroying you are slim. I'd sit Padilla because he has been inconsistent lately and is just coming off a 122 pitch count performance. I would probably start Lohse since PacBell is an extreme pitchers park.
I'd reconsider a couple of the guys you are definitely starting as well. Meche is facing the Phillies, and they are a pretty strong offense so he could get rocked. I would not recommend starting Lowe, because the game is in Pittsburgh and Lowe has a 10.33 ERA on the road this season. The Pirates offense has come alive lately (especially A.Ramirez and Giles) so he will likely get rocked to hell and back if you start him.