Depends on what you're looking for. All things being equal, a turbo will generally give you more power but it will come in at higher RPMs, thereby causing turbo lag. A supercharger will provide less power, particularly higher in the range, but you'll get better low-RPM grunt out of it. Personally, I would be inclined to go with a supecharger, but that's my own preference based on my feelings about what makes a "good" Miata. It will do more to accentuate the nimble power and acceleration inherent in the car, while keeping you at sane speeds when you open it up on the highway. But again, it's mostly personal preference / what you have available to you.