Thanks for your question. To clarify, the rules for 24 Hour Crime Scene and 48 Hour Crime Scene are almost identical, there is just a difference in how you set up the game.
Essentially, 24 Hour Crime Scene includes 24 prompts and 48 Hour Crime Scene includes 48 prompts (24 of those prompts being from 24 Hour Crime Scene).
48 Hour Crime Scene only includes the rules to set up a game of 48 Hour Crime Scene and not the original rules for 24 Hour Crime Scene.
As for the variants, 48 Hour Crime Scene includes a way for you to create and structure your own prompt deck to change the length of the game.
I hope this clarifies it for you!