That would be if Open PCI had any driverl... there is almost no driver at all. One the contrary the Mediator has much more drivers, including Terratec soundcards, TV cards, and already sometime ago they announced drivers for MPEG2 hardware decoder.
Then again if OpenPCI makes success, the Mediator probably won't be supported and you'll be restricted to the drivers supplied by Elbox. Then again IF it goes as planned you'll be able to run AOS4 on it and use it's drivers, so no loss, I don't think OpenPCI has any chance of having more drivers than AOS4.
To sum up the Mediator has currently more drivers, apparently will have many more in the future, and there is actuall support for the card. Add to that the possibility of adding a G3/4 processor and it's defenitly the more flexible option.
Then again, the price of the Mediator is very high, and the performance with the current Amiga accelerators is lower than with the GRex, due to bus constraints when talking to the Amiga motherboard. But it's expected to reach very hight speeds when you add the G3/G4 later cause then it won't depend on transfers to the amiga motherboard. Working like that it will defenitly be the faster one no chance, but only if you use the PPC processor, using the 68k the GRex will most likely be faster.
You got to decide yourself depending on what you want, and the pros and cons.
