Slash VXL battery question

[TeamLombo]

I am going to be ordering a 1/16 Slash in the next day or so. I think I am going to add the second battery. I dont understand the difference between running the batteris in "series" or "parallel". Can someone explain? Also, is it worth it to go with LiPo's instead?

[Adidas]

When you run two batteries in series, the way the wiring is done, adds up the voltage, giving you that 50MPH with a normal runtime. The paralel setup adds up the capacity. This means that you'll be running your Mini Slash at the speed of one battery, for double the time.

Series: Double Voltage
Paralel: Double capacity