User blog comment:Emptylord/Season 3 - PBE Patch Notes/@comment-79.191.107.217-20121118220914/@comment-4270771-20121119133230

it will likely proc neither. Overall the changes to it trade MR for more damage on the sheen proc. And yes it will be more damage, since it will scale off your mpen now, where before it scaled off all that arpen that AP casters have.