I am curious of this as well, right now when I get home from work I'm going to be testing this out to see which I get more output from with my Lash/Talons/whatever.
In theory I feel like spell resist would do more since you are bypassing the "raw" mitigation of spell damage that someone has (from what I understand), but investing in Expert seems as though you are just adding extra damage that can be resisted rather than actually be effective.
But then again, I am a scrubby DK so keep that in mind.

Posted his on reddit to, most people there seem to think Spell Erosion is broke and if you put more than one point into it, it stops working