whats better J or X heads

The X always seem to have a little smaller chamber, probably a smidge more compression.
The J head as a rule I believe had hardened seats in particular the later ones.
I always like starting with a 1.88 valve head as you get a fresh seat to work with.
I have a set I’m thinking of wrapping up for a 340 build, kind of want a 2.05-1.65 but probably easier 2.08-1.60
Anyways I’ve got them pretty much ported and ready except for 3-4 holes in the intake ports I will have a master probably braze.
Great head up to around 500hp