I see the two as quite different. To me, "kind" implies taking time and effort for someone, investing in them. That might mean engaging in conflict with them,

I see the two as quite different. To me, "kind" implies taking time and effort for someone, investing in them. That might mean engaging in conflict with them, if that seems to be what they need. "Nice," on the other hand, suggests to me being polite, delicate, and never making anyone uncomfortable. Niceness makes no enemies and wins no friends. It benefits the nice one more than anyone else, and is principally a strategy for staying out of trouble. Kindness costs the giver something, sometimes the risk of giving offense, sometimes their time, sometimes the loss of power and influence because of making an unpopular choice that helps someone who really needed it. It is principally a strategy for taking care of others. That's how I see it, anyway. YMMV.