My review of the Grace SDAC Balanced includes a basic comparison to the SMSL SU-8. The short version of which is that both are entirely competent, but overall I preferred listening to the Grace unit.
The RDAC is a different kettle of fish … and while I found it more enjoyable, overall, than either, that is not necessarily down to it being a technically more proficient DAC (i.e. it won’t measure as well).