If they went to an all-out war, directly against one another... I think Iran would win. Just based on raw numbers and KSA’s economic dependence on oil production. But, KSA has BILLIONS of dollars in weapons that could wreck havoc on Iran. Their air power is far superior. They could launch a bombing campaign that would make life pretty miserable.
The key is: Saudi’s new boy-King. Thus far, he’s shown an inclination toward a much harder line. He seems to want to test all his billion dollar toys. And, MAYBE... he already has assurances from Trump that, if a war breaks out, he’ll get US support. That’s my belief.
Iran is the source of MOST of the current strife in the ME. We’d love a chance to take them out, but don’t want to start or lead any war with them.
Iran held Iraq to a stalemate, basically by throwing millions armed with not much more than sticks against them.