OK I present some differences between GREX 4000 busboard comparing to the Mediator 4000 ones.
- GREX can't be purchased nowadays while Mediator can.
- GREX requires the existence of a CSPPC accelerator while Mediator works with every accelerator available.
- GREX is taking advantage of CSPPC's PPC bus using full DMA on all PCI slots (even for Network Cards and Sound cards)
- GREX isn't compatible with OS4.x for classics but with MorphOS for PUP, while Mediator isn't compatible with MorphOS for PUP but compatible with OS4.x
- GREX isn't supported these days, while Mediator does and new drivers are being written from time to time. That means that you can't use Radeon Gfx cards, Spider USB etc.
- GREX can only use Voodoo Gfx cards on CybergraphX RTG system while Mediator can use Voodoos and Radeon ones via Picasso96 RTG system.
- GREX is really stable (at least I never had any problem with it) but it's as stable as Mediator if configured correctly.
Most important problem is that GREX hardware are really rare and hard to get. Especially the 4000 versions are really limited and rare. If you happen to find such a card, just be proud of what you have and forget about OS4.x.
OS3.9 with Voodoo3 just flies on the GREX and since I used both a Mediator and the GREX I can say that GREX is faster.
Also the usage of CGX instead of P96 is a pure advantage imho, cause setting custon screenmodes (like 1360x768) was a LOT easier with the CGX as opposed to P96. This is purely my opinion though

If you want OS4.1 or MorphOS 2.x (and soon 3.0) releases, you should get the systems that are designed better for these like Macs and SAM's. For using both in one machine you can try searching for a Pegasos system.
Good luck with your search.