The finer your brush pen tip, the more detailed work you can do. A firmer tip lends itself to better precision and control, but a flexible tip can achieve more effects. This is not as critical when you are using your brush pen for coloring. In this case, ink flow becomes the main concern. You want the ink to dispense smoothly and evenly, so you don't get random hot spots or pooling! 

Not only is color selection important, but how pigmented the colors are once applied. Some people want pastel colors, while others prefer bolder, vibrant colors. Whatever the color quality, if you're going to hang your artwork in a well-lit room, make sure the ink that you use is lightfast! This means that it resists fading when exposed to light. Look for descriptions that include archival or lightfast ink.

For those who like to mix different inks and watercolors, it's paramount to check to see if the ink is water or Copic-proof. You want to make sure that it's at least resistant to water or Copic ink, so your hard work doesn't bleed away into a muddy mess.