The 24" is the one you will be happy with. It has a nice rumble without being overly loud at WOT and has minimal drone at 1800-2000rpm.
A guy I work with went to get the 24" put on his Chevy 5.3 like I had on my Ford 5.0. They put the 18" on and he complained about how much more drone he had compared mine. He measure the muffler and it was the 18". He went back and the put the 24" on like he asked for the first time and it took care of the overly loud drone problem.
This is how the 24" sounds in my regular cab 5.0 with the windows up:
Here is what mine looks like mounted to the stock tailpipe with 3" pipe added to the front: