As to the OP's original question, (which if I'm reading it was about maximizing downforce, and had absolutely nothing to do with top speed) I don't think too may people would disagree that in general (considering commonly available parts for the S30) a wing has more potential to make downforce than a spoiler. Additionally, it would seem far simpler to mount a wing up nice and high in clean air to increase downforce, than it would be to get a spoiler to generate similar levels.
Of course, you could always run both. My plan is to get one of these: http://www.prestostore.com/catalog.php/azcarbum/pd2118883 then later on mount a proper wing above it if I get that serious.
Maybe something else to consider is that the car may not be that fun to drive with a tonne of rear downforce without the same consideration given to the front.
Lots of other threads in here touch on improving front lift by venting the engine bay, so that'd be a good place to start.
Dave