I think more than likely, it's just that the materials and luminances aren't exactly the same.
It is true that LightWorks is a more advanced engine when it comes to raytracing, lighting and general aesthetics.
The RedSDK performs faster, provides a good framework for using more advanced tools like the dynamic cut plane, has a different (preferred by many) hidden line mode etc.
For getting really nice, pretty renders LightWorks is better suited. (big part of why we still offer it as a plug-in) RedSDk is more of a high performance engine. Users who push the limits of either will begin to notice the differences.
That said, I don't know a great deal about lighting and rendering. Maybe some of the other guys on the forum can help a bit more with ways to improve your RedSDK render quality.