Which one will result in faster game play ?
Create a game of a smaller resolution and then have it scaled up automatically in the iPhone or Match the resolution directly ?
Is the processor power require to scale up the graphic significantly lesser then not having to deal with that at all ?
In "High quality" (one of the project proprieties), it won't make a difference due to how C2 handles graphics (resolution being more a initial value, since the assets quality will be what matters graphically).
In "Low Quality", it should be faster, but the graphics will suffer (since the window will be rendered at a lower size than the actual screen).