I have the SA8003 which was one generation before the 8004. I think the 8004 adds DAC inputs but is similar.
I thought the 8003 was very warm and very slow. I did end up modifying the unit by swapping out some caps and that improved the speed tremendously.
I think the Oppo sounds better. The transport isn't as "sturdy" as the one in the Marantz but I find it reads just about everything. I have 4 copies of Brothers in Arms on SACD because 2 of them appear as regular CD on the Marantz. One intermittently appears as an SACD and only one appears CONSISTENTLY as an SACD. All 4 are read as SACDs on the Oppo.