I subscribed to Minature Wargames and WSS for a good few years. I've cut that down to just WSS. The production quality of WSS is much higher - photography, layout, design, paper quality all better then MW.
WI is more like WSS in production quality. Not sure why I haven't subbed to it... probably because two magazines was already too many

MW has more sci-fi and fantasy content than WSS. Both of those in WSS tend to be limited to reviews and "lets plays".
However, the scenario and history-led approach in WSS makes the articles great to read in my opinion, regardless of genre. After all a scenario is a scenario, the actual ruleset doesn't matter all that much unless playing super-crunchy (i.e. overcomplicated) games like WH40K.