When in the market for a V8 I tested a friends VTII SS and then an AU1 XR8, with185kw. I found the SS to be exactly what motoring magazines have stated, powerful, quick in a straight line, but when it comes to cornering the steering it was wooden and non-communicative. The XR8 on the other hand was fantastic, not as quick, but cornering in the thing was much more enjoyable. I will guarantee that if you test the 220kw XR8, you will be left in no doubt which is the better car. As for looks, obviously it's a matter of personel taste, I drool over the XR8, but the SS just looks like another Commodore with a few extra bits.
On the other hand, a friend has just bought a HSV XU6 and I must admit it looks very mean and impressive.