Both are possible, and in some cases they are interchangeable, but the meaning is slightly different. 'Thinking of' suggests that this person (or thing or place or object) entered your mind, possibly for a short time. For example, you'd see or hear something that reminds you of a friend, and you'd say 'I saw that book and I thought of Jo'. 'Thinking about' something implies a deeper or more prolonged consideration. You think about something in order to solve a problem, for example. With regard to a person, you might say 'You know, I've been thinking about Jo, and I've realised that ..' I hope that makes sense.

They are the same and pretty much interchangeable. "Thinking about you" is a little more common, I think.