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Just like NiCD and NiMH batteries, the higher the mAh rating the longer you'll be able to run. Now LiPo batteries also have what is called a C rating. This rating basically tells you how much "punch" the battery can deliver (mAh * C). More "punch" the quicker your car will accelerate. Usually a higher C value will demand more money.
Lipo batteries typically hold their voltage better under load. You might think "why would I use a 2S Lipo over a 7-cell NiMH?" While a fully charged 7-cell NiMh battery has a voltage of 9.8V and a fully charged 2S Lipo is only 8.4V, under load, the NiMH batterie's voltage will drop significantly more than that of a LiPo.