Two different types of batteries, A and B, are discharged. The graph shows the remaining charge $y$ (in mAh) over time $x$ (in hours). Battery A's discharge is represented by a line passing through $(0,1000)$ and $(4,500)$. Battery B's discharge is represented by a line passing through $(0,1200)$ and $(6,600)$. Which battery discharges at a faster rate, and what is the difference in their discharge rates (in mAh per hour)?