Both Prince and MJ had most, perhaps almost all, of their influence prior to 1990, so not much difference between them there. I think Prince takes musical impact. MJ's '90s musical impact comes almost entirely from the Dangerous Album. Prince had a few albums with musical impact. When you add them all together, I think Prince comes out ahead. The '92 untitled album alone is close to Dangerous.
Maybe you're right about ''add them all together". Prince has released ten albums in the decade, Jackson has released two albums and plus one remix album (Blood On The Dance Floor).
I think the biggest difference between the two is in popularity, where Jackson wins with a good advantage.