I dont know...
What I DO know is, that this has been discussed since the early days of sailing, surfing, windsurfing, kitesurfing, hydrofoiling and so on.
Does anybody have some tests confirming one or the other ?
no way to know for sure without knowing all of the details.
there has certainly been an extensive amount of research in this area with documented results under 'controlled' conditions. It's interesting to note that you will find few experimentalists that endorse a rough surface as opposed to a smooth one. Riblets are typically 'smooth channels' aligned with the flow. ' theoretically' they are intended to minimise the lateral spread of turbulence in the boundary layer.
Its my assumption that riblet technology is similar to sanding. However, I have no specific reference you would consider controlled enough to draw definitive conclusions.
A revealing reference is a compilation of experiments edited by Dennis Bushnell and Jerry Hefner;
"Viscous Drag Reduction in Boundary Layers";
Most book sites (Amazon, Google etc) will give you a preview of any section of interest.
details related to riblets are, as you surmised, dependant upon
width of groove,
depth of groove.
surface roughness of groove interior.
laminar or turbulent flow.
the list goes on, but they do work, if applied correctly.
results exceeding 5% skin friction reduction were reported
the list of 'competing' approaches to viscous drag reduction is extensive.
boundary layer heating
but the best all time is polymer injection. (>30% skin friction reduction in sea water with polyOX)
devil is in the details.