The cat back will replace everything from the headers back. Which would include taking the resonator junk out. Your cat back would either come with an x pipe or h pipe to replace that resonator. Axel back is just that. Pretty much just replaces mufflers and tail pipes. Just a sound thing.
The axle back for the 2015 require you to cut the factory exhaust for the installation. The cat back will be a bolt in installation and replace the exhaust system behind the catalytic converters
Here is the factory exhaust system as removed in one piece: (note the big bulky resonator. Where the welds are just past the hangers is the location the axle back will have you cut the exhaust system
This is the Magnaflow competition cat back system that I installed on my GT:
This is where the exhaust runs under the axle:
This is a good look at the X pipe and where/how it attached to the factory exhaust:
FWIW, the factory exhaust is 2 1/2" and this Magnaflow setup is 3". I am not sure if the axle back kits increase to 3" or not. I saw an increase of 10 rwhp with just the exhaust system, not to mention a great new sound!
Good luck, let me know if you have any other questions