I fail to see how one person can believe that both of these are true. Of course, if you feel that Christianity or some other religion is the path for you, that doesn't have to stop you from reading and learning more about Witchcraft, or any other religion, for that matter. You can only benefit yourself by learning more about other paths. It's not evil or a sin to open your mind and get educated about the world around you. That's kind of like asking what the difference is between Baptists and Christians.
Paganism is a word used to describe a very broad range of beliefs.
Witchcraft is sort of a denomination of Paganism. In other words, most Witches are Pagans, but not all Pagans are Witches. Some Pagans follow Asatru or Druidism or another religious path. You won't find us passing out tracts on street corners or knocking on doors spouting sermons. Wiccans do not try to convert others to their faith because we believe there is more than one path to the God and Goddess.