I have used a grex1200 with my rev2 060 BPPC and now a G-Rex4000D . _never_ had any prob and I highly recommend them.
I have also used a mediatorZIV with the same BPPC, and I liked the grex better (better speed and compatibillity)
ofcourse the g-rex has the advantage of interfacing with the accelerator directly...
so for me, the G-Rex is a winner . You don't state if u use a BPPC or a CSPPC... u might not get the g-rex1200 working OK with an rev0/rev1 bppc (most of phase5 bppc's are that rev)
also the rtg software (picasso96) installation on mediator is at least LAME.