Games like Call of Duty 4 run at a framerate of 60 frames per second on both systems. But Call of Duty 4 is a game that experienced technical problems with various bottlenecks due to the varying levels of action that occur on the screen. The game was originally designed to run at 60 frames per second at 720p, but it caused drops in the framerate.
The proof of this is the fact that Call of Duty 4 runs at a resolution of only 640p on both the Xbox 360 and Playstation 3. This may be "equal" in terms of what is displayed on the screen, but the FACT is that Call of Duty 4 is putting a much larger burden on the PS3 than it is the Xbox 360.
Confirmation of this can be seen in the large number of games that "run smoothly at 60 frames per second on the Xbox 360, but struggle to run at 30 frames per second on the Playstation 3." That was a quote from Gamespot.
It's simple math: The Xbox 360 can display approximately 500,000,000 polygons per second, compared to 275,000,000 polygons per second for the Playstation 3.