The G-Rex 1200 sometimes gives more problems than the 4000 version with certain boards (and is less compatible too), and some people has toasted their ppcs... although the number of drivers is similar, you should remember that OS4 probably won't support G-Rex... If you plan to use MorphOS then it's no problem. The price difference is important... if it was supported by OS4 I would buy it, but with the current situation it seems unlikely that we see G-Rex supported...
An added problem is with the G-Rex is that if your ppc dies you'll have to buy another expensive ppc... in contrast if you use a mediator you can plug in any turbo card.