A catback exhaust usually comes with mandrel bent tubing which keeps it's overall internal diameter even through the bends. your stock piping and any pipe bent up by your local muffler shop is compression bent and will shrink during bends.
If sound is all your really going for with maybe a smaller boost in performance you could get just the muffler and get a slightly larger diameter exhaust system bent up by a muffler shop, but you will lose some power in comparison to the mandrel bent stuff.
It's really up to you, i've got the muffler/custom bent pipe setup currently getting built on my car, but it's cuz it needed an exhaust right away. but given the oppurtunity where i had enough money to spend i would've gone for the mandrel tube. The choice is yours, so it really depends on what your needs are right now.