Guendouzi is not on Ceballos's level............. there is absolutely nothing Guendouzi can do better than Ceballos. He doesn't defend better, doesn't pass better, doesn't play with more intelligence or creativity, doesn't have more strength, doesn't perform more consistently..........
Ceballos doesn't really do anything of the things you mentioned to a high standard either. He's had a handful of good games since being here, there isn't much in it between the pair of them.
It's not really that relevant as neither are starters, or should be considered starters. Both are fine as squad players, so go with the cheaper option.