It depends on what YOU want and how much money you have.
A full system is always better power wise. But then you should get a power commander and some tuning to make sure everything is running correctly.
A slip on is a quick and easy upgrade to increase your exhaust flow a little bit because they are more free flowing than your stock exhaust in most cases. Other reasons people do slip ons are because it's an easy way to add a new look and/or sound to your bike.
I personally went with a slip on since my bike has no catalytic converter, so to me it made more sense to go with a slip on for now since my stock headers are titanium. Plus, the pipe I wanted didn't come as a whole system. So I decided to just stick with a slip on until I actually need to worry about every little bit of power and so on.
But like I said, in the end it just depends on how deep your pockets are, what you like, and what your purpose for changing the exhaust is...
hope that helps some