I don't like either option too much at this point, but you've got to go with Padilla. Robertson showed signs of brilliance earlier in the year, but he's definitely hit a wall lately. With his rookie arm, there's no way we could expect his numbers to last well into August and September. He faded, just like all rookie pitchers do in the last 2 months of the season.
Padilla, coming off injury, will have a rough time of it at first and will probably be limited by pitch counts for the first few starts, but he's a quality pitcher, and he should be hitting his stride just in time to help you make a push in the last 5 or 6 weeks of the season. The win potential on Philly is a lot better than Detroit as well.
I wouldn't want to rely on either of these guys, but at this point in the season, your man is Padilla...and I'm one of the biggest Robertson supporters here.
Hope this helps!
[size=10]"Men are apt to mistake the strength of their feeling for the strength of their argument." [/size]