Hi mate,
For what it's worth I'll give you my opinion on what I would do if I were you.
As has been mentioned previously, we don't get much nice weather even in the Summer, so for me that'd rule out the Porsche. I know it's a fair bit drier in the East, but arguably colder and windier too. Not the kind of weather I'd immediately be thinking about getting the most from a cabriolet .
With having the magnificent scottish countryside, and the gateway to the Highlands not far from your doorstep I'd really lean heavilly towards another STi if I were you. It's really all cars for all people. It'll be perfect for our (crappy at times) roads, and tbh I don't like M3's so that would rule itself out straight away .
Do you really need a car???? Remember you'll be a student, doing everything that students do
Good luck with reaching your decision, but it'd be the scooby everytime for me.