The main benefit of going with a full catback kit is that they are bolton which makes it easy for you to put it on and take it off if you decide to sell. The full catback kits comes with polished stainless tips, which will run you about $200 if bought separate and will require adapters if you plan to run 5" tips. The full catback kits also remove the factory resonators which gives a little better flow.
In the end, a catback kit isnt necessary, but if you plan to replace the muffler and install 5" tips, then going with the complete kit isnt a bad idea.
If you are just looking for better sound, then the muffler swap is perfect.