Use my US Passport or Taiwan Passport to enter Taiwan?

I plan on attending school in Taiwan to learn Chinese sometime later this year (maybe June, or September). I would like to go to Taiwan early (27-Jan) for Chinese New Years and stay there until school starts. I will be staying with my parents who live in Taipei.

I am 28 years old. Born in Taiwan but left for the US at the age of four. I have both a US Passport and Taiwan Passport. Which should I use to enter Taiwan? I would like to avoid military service if possible.