Well the FireGL 8800 series of chips support more memory. (They also run at a faster core/mem clock, but that's beside the point)
The drivers are ISV certified and are designed by the FireGL driver team, and so the driver base is radically different then that used for the typical Radeon cards, the driver base also contains many application specific optimizations and profiles.
Also, the FireGL 8XXX supports a number of advanced features the Radeon series lacks, such as hardware accelerated lines/points, multi-sided lighting, enhanced memory fetch buffer, user clipping planes, anti aliased vector rendering etc etc.