I'll say Kane now and in the future point total wise. (though I agree Kopitar brings more to the table as a more complete player)
But it makes it more difficult to decide and is intriguing when you consider that Kopitar is on nearly the same pace of production to a man of similar statute who goes by the name of Malkin at the same age and he doesn't have someone on his team that posses anywhere near the talent of Crosby.
That being said it is very difficult to ignore Kane's current and future offensive ability.
just considering their 1st season, Kane wins the poll pretty handily. an 18 year old beating a 19 year old in points and winning a Calder, Kopitar wasnt (albeit he should have been over J.Staal) nominated. However, Kopitar had a tremendous follow-up season, which Kane will have next year. This will be a better determinant, but as for now, because Kopitar has show that he can produce for 2 season, he has to get the edge, now and future
its very close but i would go with Kopitar since i think he is going to be the better overall player, Kane will likely have more points but id rather have Kopitar on my team, i think he will be a better contributor.