As pointed out by Agirre et al (2009) and Levy & Goldberg (2014), relatedness may actually be measuring topical similarity and be better predicted by a bag-of-words model, and similarity may be measuring functional or syntactic similarity and be better predicted by a contextwindow model.
comparing FW-SG with SW-SG and SW-cBOW, there is almost no sign of performance gain from training using full Wikipedia instead of the much smaller Simple Wikipedia.