Let's break down the previous meetings:
Apr 26-28 @ SF
Giants win 5-1, Halladay's fifth start of the season. Sanchez with the win.
Giants win 6-2. Against Jamie Moyer. Wellemeyer with the win.
Phils win 7-6. Giants take a 3 run lead on Hamels, then blow it in the 9th to eventually lose in 11.
Aug 17-19 @ Philly
Phils win 9-3. Oswalt over Zito.
Phils win 8-2. Blanton over Cain. BLANTON.
Giants win 5-2. Sanchez over Hamels.
The first 3 meetings were without Jimmy Rollins. The last 3 meetings were without Ryan Howard. But you really can't make any judgements based on these games; the sample size is small, and the regular season isn't really the same as the postseason. In my mind, the pitching is fairly evenly matched, although I would give a slight edge to Philly for allowing 4 runs with 2 shutouts to sweep the highest-scoring team in the NL while scoring 13 runs. The Giants allowed 9 runs in 4 games to the Braves, while scoring 11. The offense will make all the difference here. San Francisco has a barely average offense, while Philadelphia, even while missing Utley, Howard, Rollins, Victorino, and Polanco for much of the season, have one of the best offenses.
Phils in 5.