Both would be best, but if I had to chose I'd go for smart. I've seen smart, but unkind people learn to be kind over time. I doubt a kind halfwit can learn to

Both would be best, but if I had to chose I'd go for smart. I've seen smart, but unkind people learn to be kind over time. I doubt a kind halfwit can learn to be smart. That's presumably the case because smartness defines your general ability to learn, change and adapt.