[puddleglum]

First, there is a lot of debate over on "endless sphere" as to which battery chemistry is better. You might want to do some reading over there before you decide on a pack. From what I could gather when I was reading, LiPo (hobby king pack) is best power to weight ratio and is best for high power bikes where weight and power is more important than range or longevity. LiPo can thermal runaway and explode if overcharged so charging must be monitored more closely. They are only good for several hundred charge cycles but the price is cheaper than LiFe. LiFe (Ping pack above and similar) are best when power output is less a concern than cycle life and easy maintenance. Eg. a longer range, lower power setup. It is a safer chemistry so requires less monitoring when charging but most packs are only good for 2c-3c max so you need a bigger pack to get high watts and they're more money. On the up side, they should be good for up to 2000 cycles, so cost/cycle is less.
Also, if you are talking of wanting a motor that can propel a possible 500lb load at 15-20mph, I think you need to go rear drive, 48 volts and the most wattage that local laws allow for an ebike. Get a big battery pack too.