What brush should I use for abstract art?
First, the fact that you are considering using a brush at all puts you far back in art history. Ever since the 1950s, with the art of Jackson Pollock and other abstract impressionists, applying paint to a canvas could include dripping (direct application using gravity and the natural qualities of liquids). An important small film called “ Day of the Painter” (1960) showed the world how that technique worked.
As for your paintbrush choices, there is no "should" in artistic expression. Much of your decision depends on what you want to do with color and light, what you want to “say” with your abstract piece. A painter in the twentieth century would intentionally exaggerate his/her brushstroke, because that kind of artist was interested in displaying the act of painting. But earlier in art history, that was not the case.
Before the invention of photography, the most important purpose of painting was to depict and preserve a real-life image (a portrait, a landscape, an historical moment, etc.), but in Modernism, the goal was to create a visual image that had not existed before that painting existed. Consequently, the use of a broad series of bristle brush sizes and textures was necessary to recreate reality, but in Modernism such tools as wide, flat brushes (and trowel knives) served well.
Mark Rothko, in his abstract expressionist, beautiful, almost spiritual canvases of rectangular monochromatic contemplative pieces (1940-1970), smoothed out his colors in such a way that brush strokes were virtually invisible.