I'd go with an Enerland cell based pack such as Hyperion, Polyquest, Flightpower, Trakpower, Thunderpower, NeuEnergy, etc. These are the best batteries available on the market right now. I'd go with a 4s pack for racing, and for speed runs you can run up to a 6s lipo, although you should watch temps when running 6s and you will probably need to change your gearing as well...