The J and the L are the exact same airframe. The only difference is the huge increase in roll rate above 375 mph. Also you have dive flaps as well. Note that the dive flaps DO NOT create enough drag to actually slow down the aircraft. Its sole purpose is to help keep the nose up during dives. All other performance specs, including climb, speed, horsepower, turn rate is all the same.
Having flown the P38s a lot this past year, I can say that the L offers more options in a dogfight than the J. People generally fly the J because of its skins, and the ENY value.