Mainland China is controlled by the Communists whereas Taiwan is controlled by

I believe the answer is: Nationalist

Taiwan and China are involved in power struggle regarding Taiwan's independence status.

During the communist Era, Taiwan was indeed a part of Chinese government and set aside as a special region for international trading.

But now, the nationalist controlled the country which advocate for separation from China.

The correct answer is Nationalists

Taiwan is controlled by a nationalist government that forms a unitary semi-presidential republic