Both shows are options.
If you go to the second show, it usually isn't as crowded as the early show and you can do rides while a large number of people are in line waiting and at Fantasmic. And after, many leave the park rather then going back for rides.
If you do the first show, you need to be in line 45 minutes before the show unless you have a package for it.
After, many leave, so lines are shorter.