The G-Rex is faster but limited in drivers and no longer made or supported by DCE, and if your PPC card dies then you can't use the G-Rex any more.
Mediator is still made and supported and will work on pretty much any Amiga with a turbocard from an 030 upwards.
Drivers installation is a bit of a chore on either. G-Rex gfx drivers use CGX (like MorphOS) and Mediator uses Picasso96 (like OS4), although there are CGX v3 drivers for Mediator + S3 Virge and a Beta CGX v3 driver for Mediator + Voodoo3 floating around.
The best Voodoo PCI card for the Mediator is the Voodoo 3 3000. I have used a Voodoo5 in an A1200 + Mediator and it didn't run any faster (actually according to P96speed, certain benchmarks were slower), and the Voodoo drivers don't take advantage of the 2nd GPU on the Voodoo5.
