I am a middle school student. I want to practice my writing style. Which copybook is better?

It is very important to learn calligraphy and choose copybooks! You are a middle school student. I suggest that you study Wang Xizhi's Theory of Music and Buddhist Scriptures first. These are very classic masterpieces of regular script. After you have laid a solid foundation, you can write the preface of Huairen Sheng Jie. This is a very formal way to learn calligraphy in ancient and modern times. To learn calligraphy is to learn from ancient times. The ancients said: "Law goes up, only follow it, and law goes down."