With 3.79" stroke your rpm range will be about 350 higher, and with 4.125" stroke 200 lower. What is best for you depends what rpm range works best with the rest of the combination. Your chassis and the track you race has also a great meaning to this. If you can handle more torque without hooking issues you will most likely be faster with more sroke, assuming the rest of the combination is optimized for that stroke. However, the difference might not be much, and your money could be better invested in somewhere else.