Arcane Mastery vs Spell Penetration which is better to consistently overcome SR?

Spell penetration gives a +2 to overcome spell resistance.

Arcane Mastery allows you to take 10 on a caster level check to overcome spell resistance.

You have a 55% chance of getting a 10 or higher when rolling a d20.

And most monster SR’s are fairly low… which feat will more consistently land?