Well I think the argument here would be that the one who invests more time will have read/compared more posts, giving him an advantage when evaluating which ones he should invest the most voting power in. In both cases the optimal move is to upvote 5 posts (due to superlinearity). If you spend more time curating, you increase your sample size and are more likely to upvote only good content.

I agree that the edge one gets from spending more time on the site is most likely diminished, but it should still be there.