Both reading and watching television are sedentary occupations. However, the level of mental activity is considerably different. All else being equal, reading is considerably more active than watching television, because reading is a cognitively active task. When you are watching TV, you are passively receiving images, in an order controlled by another person. What you see is what you get. In contrast, reading requires you to imaginatively bring to life what you are reading. Even the most basic, undemanding form of reading requires decoding, the transformation of printed letters into meaningful words, and as such is more active than viewing a TV program. Furthermore, readers are in more control of their activity, able to slow down, speed up, skip, and re-read as desired, something that they are unlikely to do while watching TV.
So: all else being equal, reading is better for you than watching TV.
But there’s a social variable that makes a difference.