depends on your budget. the slip on will help you gain some hp, but the full system will gain you some tq in addition to hp. either one, coupled with a power commander will significantly improve your throttle response, appearance, and sound.
A full exhaust is the best choice for performance if you have the money. Bolt-ons don't give you the added hp that a full system does. The full system headers and mid pipe will have a larger diameter thus flowing more air. Remember, you must get a Power Commander for the adjustment of the fuel injection for a full system. You can get away easy with just bolting on a can and nothing else if you want to but real performance power takes a bit more work and money. A bolt on may cost you around $300 bucks but a full system + PC will cost you around $750 + or -