I had a 2001 Crown Vic (4.6 SOHC), and looked into this awhile ago. If you are going as far as getting a turbo or supercharger, that right there is pretty much the best bang for your buck. Of course you would do well to upgrade EVERYTHING else as well - you don't want your new power to tear your car apart. That means forged internals, upgraded tranny (+ torque converter w/ higher stall if automatic), driveshaft, differential and maybe even rear axles. Don't forget all the other things that go with forced induction: higher pressure injectors, new intake manifold, new cams & springs, etc. Maybe even shock tower struts to resist frame flex (or are 'stangs unibody?). In any case, getting a turbo or supercharger will require you to do a lot more than just install it. Pretty much the whole driveline must be upgraded. You'll need a good tune too.
As for as super or turbo it goes something like this (I think). Turbos work off the exhaust gas flow, which spins an impeller which drives a pump that forces more air into your cylinders. A supercharger does the same thing but uses the accessory belt instead. The overall idea is the same: more air = more fuel = bigger bang. I would think a turbo would be better for a smaller engine - like a civic. They rev higher and thus push the exhaust air out faster (spinning the turbo faster). Besides, most small engines don't have the torque for a supercharger to be most effective. On a larger engine (like yours) there is much more torque available, and the exhaust isn't going out as fast anyhow - a supercharger seems like a better choice. They are also easier to install and tune.