The transmission in the XR-2 is the strongest of the two being as it can be made with better internals, and a LSD. It has some bad problems with twisting from high torque that the LSD fixes.
It has a weak differential, which the LSD fixes. Once you get into higher powers (idk, 300,350+) you still have case flex issues which causes 3rd gear teeth to shear off
Look on Clubprotege. There's folks who use an adapter plate to run a Toyota transmission. The axle combo works on BG cars so you will have to get creative to run it in a BF