just wondering which is better and why, i know the difference, but dont understand why one is better.
see im under the impression supercharging is better for 8's while turbo is more for 6's

Either one could be better or worse depending upon your application, what turbo/supercharger you use etc etc.

The I6 work well with turbo's as you generally don't suffer many lag problems and is pretty straight forward to fit. Positive displacement chargers are a lot harder to fit to i6's and a lot easier to fit in v8's which is why they are commonly used in 8's (as oppose to twin turbos taking up space or single turbo with a huge manifold).

As it is an fg and pretty new, why not upgrade to an xr6t? You will probably spend as much turboing your car, plus your resale will be a lot better with an xr6t. If not your next cheapest/best option would be to get an xr6t engine out of a wreck. If you intend to go aftermarket you will get a*** raped for $$$ by the time you pay engineering which you would want to do considering the cost of the car (insurance in case of accident).