The overload only works on your nukes, and since our number one killer now will be a fully charged earth shock, since its a 10k instant cast that crits for 20k, I think that 8% crit [8% chance for double damage] in exchange for an extra 16% for an extra 0.75 damage nuke (not including the earth shock release) is fairly equal.

All in all, at this stage, it seems like preference. Increasing the crit chance will make your earth shock crit more when chasing down flag carriers, and point for point, the mastery will just make your hard cast nukes hit harder.

For me, I think I'm gonna stick with crit, but thats just because I like a 45% crit chance with my hardest hitting ability.