They're not the same. The Sphere Grid allows you to actually customize your characters if you so choose, by erasing spheres, adding spheres to the grid, using key spheres to break into other sections of the grid, using friend, teleport, and various other spheres to allow further customization. The crystarium is literally hold X to advance along six predetermined paths and it's really only three considering the majority of characters are horrible with their secondary three classes and they take way too long to develop. Any branch leads to one or maybe two abilities or stats. FFX's level up system is simply unarguably better.