Maximum Hit Points

[pawsplay]

As for monsters and CR, for low CON and high-dice (eg d12) monsters max hp might be worth +1 CR. If they get most of their hp from CON then it's not a big difference, but doubling hp is certainly worth +50% XP as the monster will be around nearly twice as long, doing nearly twice as much damage.

That's not necessarily true. They are also around longer to get debuffed, and there is often a fair amount of blow-through.