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

Possibly closer to what the OP was asking; back in the late 80's my DD truck was a 78 W100 power wagon that came stock with a thermoquad equipped 360. When I got the truck it had a cracked head. I picked up a pair of heads locally for $150, cleaned them up and put the motor back together, and drove the truck for a few years. The motor lost compression on a few cyls , so I swapped the heads out for a pair I bought off a friend. I drove the truck for 3-4 more years then sold it to a friend. I even threw in the old heads I had taken off years earlier. Later the friend tells me that the heads were X and J heads. I was never able to tell any difference between the power the truck had with either set of heads on it. I raced other trucks in the mud and never lacked power with either set. Both were unported.