When you get the choice to go to Mushroom Rock or Bevelle, you must be aware of the consequences of each. Each scenario basically just results in a lot of scenes and the obtainment of Leblanc’s Sphere . However, the consequences go beyond that:
Mushroom Rock: This implies you will join the Youth League’s side, and will also make 100% completion possible. It will also open up a Chapter 5 sidequest. However, you also lose several things - access to Bevelle in Chapter 2 and most of Chapter 3, the chance for the Blood of the Beast and Shining Mirror Garment Grids, and a certain Chapter 4 sphere in Bevelle. This is the option for those attempting 100% completion.
Bevelle: This means you’ll join alongside New Yevon. The main problem with this is that it makes 100% completion impossible . However, you will be able to get the Blood of the Beast and the Shining Mirror Garment grids, and also a certain sidequest in Mushroom Rock in Chapter 2. Then again, you also lose access to the Youth League’s area in Chapters 2 and 5, and lose a certain sphere in Chapter 4’s Mushroom Rock.
After the scenes in either area, you will basically be required to get three Syndicate uniforms by stealing them from their wearers. A slightly odd mission, we know. A hotspot will appear in Chapter 2 - Guadosalam (Mission). For now, though, we’ll skip that in place of other things.