Immortal male leads chase other immortals all the way from the ancient past into post-apocalyptic dystopian futures, armed only with their swords, and find themselves involved with resistance groups. In both love is the driving force but where they differ is Highlander's driven emphasis on revenge as its lead, Colin, gets his arse kicked for 1000+ years by his immortal arch-nemesis, Marcus: the man responsible for killing the woman he loved. Kurozuka just has its lead fight people as he desperately searches for his immortal woman; trying to unravel the mystery of what happened before he woke up in a bleak future.
Madhouse created both titles. Highlander has Kawajiri's distinctive action flair and is far more focused as a movie than Kurozuka is as a TV series. In fact, Kurozuka is all over the place after its two excellent / stylistically-gruesome first episodes.