Offensively, I feel they are roughly equal with an all so slight edge to Kane.
Defensively, Kopitar hands down. Second on his team among forwards in pktoi/g and gets a decent amount of points shorthanded.
Physically, neither are exactly bruisers. But Kopitar is much, much, bigger. You can argue whether or not size matters until you are blue in the face, but I don't see how being 6'3 220 can be a bad thing.
I also consider 1st line centers to be more valuable than 1st line wingers.