Supercharger is better if you want to be quick off the line and get some decent boost at higher RPMs. Turbo will be slower of the line (turbo lag), but will usually provide more boost at higher RPMs.
For "cheap" mods, CAI, true duals, ported engine, new cam, new chip, good headers, etc. The super/turbo alone will cost at least $2000 for an "ok" one, upwards of $4000-$5000 if you want a really good one. CAI and Exhaust will be about $500-$600 depending on brand. You'll need new tranny and gears to handle the super/turbocharger. Get suspension mods as well for better handling and less body movement.
BTW, GT's aren't that quick. I kept up with one from a 30mph roll, up to about 98 (when we started slowing down), he had maybe half a car length on me.