The Camaro has much better suspension geometry out of the box than the Mustang, obviously the SCCA factored this into the minimum weights as the Camaro is heavier. From what I have read what is needed in order to get the Mustang to handle is illegal for IT anyway, you have to relocate all the rear suspension mounts. Engine power will be similar I would guess, but based on the handling alone I would go Camaro, there is also a large aero advantage, the Mustang is about as aerodynamic as a brick. And then you have bigger brakes on the Camaro too.
IMHO Camaro wins, hands down.