On my staff, my DPS goes down when I put 2 soul cores of critical dmg in it instead of the 2 iron runes. Which one actually does more dmg? I'm fully a critical build but dps shoes less with them in.

Theres no clear answer to this. All the variables that impact your damage can change the answer. You can check PoB2 to see if it can answer your question. I dont know if its ready for this yet.

But you either need to know damage calculations and all the variables on your character, to trust the dmg sheet, or test what feels better.

Too many variable to take into account we have zero info beside the runes

I believe crit does double damage as a base. Then crit damage bonus adds to that. So essentially double your normal dps and see if tha number will be higher than added dps. If anything increase dps and crit chance. Higher chance means higher damage more often. And if im correct and thats the way crit works just increasing dps and chance would be significantly better. Someone smarter than me can fix my mistakes or affirm it. That is just my opinion.