Would just buy a turbo or super charge car. not worth the money in the mods .
Which if you actually read what the op has posted is NOT what he is asking.
It's an either or question, not what would you do instead...
Personally I would drive both and see which fits the bill. I've never been a fan of turbo's and hate the whiney chargers on the Mini's, but a Rotrex sounds particularly nice and is fine with standard clutch, box and engine and does not have any reliability issues (well, mine certainly doesn't). I know this first hand, anyone saying otherwise is quite simply wrong. My car has had pretty much every modification available at some stage and is living proof that if things are done properly to a high standard then there wont be any issues. The people spouting otherwise are the ones who have never had the balls or incling to do anything out the ordinary themselves, or took it to some cowboy outfit and got hosed.
My car has always been looked after and had all it's work done by Andy @ GDI (now K-tec) and proves my point quite nicely... quick run down:
36k - cams and SMT6
year later - ITB's
few years more - bigger ITB's!
present - charged and on 110k with no issues, it's the most reliable car I have ever owned.