OK, here we go. A stock 4.6 ford internals can handle up to 500 HP. Not sure about the LS. But, the Mustang is historically lighter than the Camaro, so that is advantage Mustang. Think about an '80-'86 Fox body with a turbo, destroys any Camaro from the same time frame.
Can't remember who, either DC Power or Mechman, had an old fox body they built up.
I don't think the OP's talking about new cars, but even if he was, the 5.0 Mustang kills the new Camaro, even with the solid axle.