I'm in the process of helping someone select a Dell Inspiron 15 and I'm wondering about the physical size of the battery. The default is a 4-cell which is probably just a few hours of battery life. Then there's the 6-cell for an extra $35 and 9-cell for +$75. I think the 4-cell is flush on the back of the laptop but I'm wondering, does the 6-cell stick out in the back or is it also flush? I assume the 9-cell certainly sticks out. I'd prefer one that's flush and I'm really curious about the 6-cell.