The 250 Series Kit can be installed in as little as 5 minutes so it's for the rider who wants to get outside and ride as soon as possible. The 250 Series Kit is simple, reliable, and will give you ample power to speed up hills with very little effort! The 500 Series Kit provides more hill climbing power for the steepest of hills and torque to move heavier loads. The 500 Series Kit is perfect for heavier riders, fat bikes, trikes, tandems, and more! Each kit is hand built in the USA customized for each order! If you need help determining what sizes and options you need please give us a call at (866)933-8716! At Leeds, we pride ourselves on our expertise and customer service.