Does anyone have experiences with both Castrol SRF and Prospeed GS610?
How did they compare?
Any thoughts?

Brake fluids are most easily compared using the published wet and dry boiling points.

Castrol has the upper hand in wet boiling points.. and the GS610 has a slight advantage in dry.. however, which fluid is more hygroscopic?

Generally a lower wet boiling points means the fluid is more hygroscopic, IIRC.