A Wing fence also (if i can remember my Aviation Technology Lectures) prevents "Ram's Horn" vortives from forming on the wing. These spanwise vortices destroy lift and increase drag. A wing fence stops them from growing past where the fence is.
As for a winglet, all i know is that they control the wing end vortex (the one caused from air spilling over the wing end) and increase lift and reduce drag.
Matty