Its like comparing apples to oranges mate. XR6T is in a whole different class. 350rwkw out of a XR6T would still not be as fast as a 250rwkw GTST around mallala. If your going for outright speed and power then modding the XR6T off its head is where i would be going but if it is going quick around mallala then GTST is where my $$ would be going.
You have to decide what you want out of both of your cars mate. In the end you would prob make 350rwkw out of your XR6T and drive happily not really worrying about coping a defect as well.