X-heads vs J-heads: what's the difference??

i know more about body work and paint than engines but i will say my original (1968 owned in 1969) 340 4sp fastback s had x heads and my current (mid-life crisis) 68 340 4sp fastback s has x heads. i have ALWAYS been happy with how these motors perform on the street. obviously, heads are not the only part of the story, both my cars had/have 750dp holleys, headers, high rise manifolds and clutch fans. i have a 68 340 on a stand that i am currently building and i will be using edlebroc aluminum heads, aluminum water pump, aluminum manifold and a "mini" starter. the old adage in drag/street racing was/still is true - "the lighter the faster." last i read, swapping two x heads for two aluminum heads will drop 50 lbs from your car. adding some other "weight loss" mods can improve performance noticeably.

yes, none of what i just said addresses your original question of which head is better - an x or a j. since i am an old school guy, i prefer the x head.