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

A young 'Snake' Plissken and Elvis discuss the differences between the 'X' Head and 'J' Head.

Elvis said he liked the 'J' Head, because of a slightly better flow at lower RPM's with a moderate-lift
Camshaft {for Street-Driven A-Body cars}.

'Snake' is about to drive his right foot in to Elvis' right ankle.

'Snake' says....................'X' marks the spot Elvis.