Alex - great question. The 458 Challenge is significantly easier to drive on the track than a 458 street car. The suspension is better set up, the stability/traction control is better designed for high speeds and intervenes less, there is more traction, the ABS is less intrusive, and the seats etc. hold you in place so you can feel what the car is doing. If you do make a mistake or get a slide, fixes are quick and easy, and the suspension doesn't load up and require multiple corrections. The car is not designed for a complete novice to the track - for example, if you whack the throttle open while hard in a turn, you will oversteer - but if you can pilot a car with no stability control at 8/10 around a track, you'll find it easy to drive the 458C at 9.5/10.
Compared to our 599XX, the 458C is less powerful, has less grip and significantly less brakes, and is more nimble and more forgiving. The 599XX has a more intrusive stability control at the higher settings - it's designed to keep a novice alive with >>GT-class power and grip! - and more drama when you drive it. Side by side I expect the 599XX would be quicker at a high-speed circuit, and might be a toss-up on a slower circuit.
Cheers, Ben ( Too many toys, not enough garage space! )