The most recent training data that I've seen shows that JS Wingmen gives you no training in IS and higher training in JR/DR/HA compared to JS Forwards. JS Guards, however, gives you much better results in all of the categories that JS Wingmen trains, except that there's a small bit of training in OD listed for JS W, which might be a result of cross-training.
If you want some IS training in exchange for giving up some JS/JR training, then JS F makes some sense. But JS W just seems horrible, compared to JS G. Unless you really need to get your players training at SG/SF specifically, I'd avoid it and play my SF out of position.