what bike do you have? carbed or injected? If its just carbed, you can get away with either full or slip-on exhaust. Injected, you won't get much performance out of it unless you get a tuning computer system for it and get it mapped.
If your looking for more performance, get a full exhaust. But, most guys on bikes don't get exhaust just for performance, usually they want a sick sounding bike. More power is just a bonus. Btw, if you have a new bike, slip-on is good since stock headers and pipe are almost the same as a full exhaust system.