I've owned a RZ TT just after an BA XR8. I now have a Mk2 GT but have not owned an XR6T (driven quite a few & F6's). The RZTT's standard have around 180 rwkw's and the XR6T's about 190 ish. The XR6T's have much more torque however and feel stronger down low stock (RZ is stronger mid-range & top). RZTT is quite refined and is more flexible standard than the XR6T's. More noticable turbo lag in the Supra compared to the Ford and there's also a torque hole between turbo's coming in which can be annoying. Supra engine note sounds much sweeter to me than the XR6T.
For outright pase the stock RZTT will be faster than the XR6T (400m times range from 13 to 13.6 for the RZ compared to 13.8-14.2 for the XR) and handle much sharper as well. Biggest difference is in the 0 to 100 times where the RZ has a power to weight advantage capable of up to a second faster than the XR.
On the other hand the XR6T will feel much more comfortable and be more easy to live with. My RZTT was fantastic but became tiring to drive everyday. After 6 months I would have happily swapped it for a good XR6T.
Getting parts for the Supra is a pain as well and electrical..forget it! Both have huge performance potenial!!
Both cars are like chalk and cheese really.