First: writing tools are different. Brush is soft and thick, and pen is thin and hard. Calligraphy written with pen is called "hard pen calligraphy" to distinguish it from traditional brush calligraphy.
Second: writing a paper is different. The pen is written on bright machine-made paper with close fibers, and the brush is written on Xuan paper, wool paper or leather paper with slight infiltration effect, which has charm.
The biggest difference between the two is the use of different functions. Brush calligraphy, because it is more troublesome to carry and use, has not adapted to the needs of the development of the times, and has gradually withdrawn from the practical category and become an ornamental art. The pen is light and can be written instantly at any time and anywhere, even a well-connected computer is far behind.