In the market for a new exhaust for my 08 sport. Is it worth doing a cat back system or just a muffler? Any differences between Gibson, Flowmaster, and Magna flow other than sound?

The Libby exhaust is already mandrel bent stainless. So the only advantage to doing Cat back is to avoid an extension and or clamps. If you weld in a new muffler and an extension (if you go with a shorter muffler), cut out the resonator near the end and add a small piece of pipe, then there is no functional difference other then the weight of your wallet.
Me, I removed the OEM muffler, added a Magnaflow #11226 with about 18 1/2 to 19 1/2" long extension with clamps(bolts pointed up so they would be less likely to catch on anything). I chose to leave the resonator on.

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07 KA Light Khaki, Manual trany.
2.5" Adjust-A-Strut JBA lift, Euro taillights,
Home made; Front Tilt Hood, Roof Rack, RAM Air, & CAI.
ESP Off Switch
Take a ride on the Trō side!

Muffler it is. thank you

Your welcome. I forgot to mention I put the extension in front of the new muffler. I did this to locate the muffler toward the rear which reduces cabin drone. Muffler is 2.5" Center in / offset out.

_________________
07 KA Light Khaki, Manual trany.
2.5" Adjust-A-Strut JBA lift, Euro taillights,
Home made; Front Tilt Hood, Roof Rack, RAM Air, & CAI.
ESP Off Switch
Take a ride on the Trō side!