whats better J or X heads

Back in the day, you could tell the difference in power with the X heads. Not much but you could tell. That is why they were prized. No other reason that I know of. Both are great heads, 1.88 or 2.02. X heads only came on 340's. J heads came on any 360s and 71 up 340s. There are/were a lot more J heads out there.