If top speed is your concern, battery isn't the best place to look.

The difference between NiMh and LiPo are in chemistry, and physical makeup.

The part that matters to you, is internal resistance. LiPo batteries have far less internal resistance than NiMh, this means there is less voltage drop under load.

Also, the discharge curve on LiPo batteries is flatter. A fully discharged LiPo is 3.8v, instead of .9v for a NiMh cell. The NiMh cell loses 1/3 of it's voltage, while the LiPo loses only 1/5th. This means your car slows less before the battery dies.

NiMh cells use Nickel as one of their electrodes. And they use metal cans to support their cell structure. LiPo batteries use lithium as their electrodes, which is very, very light, and they're enclosed in plastic, which is also light. Which means for a given physical size, LiPo's are lighter. For a given capacity, they're smaller.

What you didn't touch on, was what was your goal?