You ask very good questions. And of course, there is no easy answer, or you wouldn't be here asking, right?
I am not inclined to spend a whole lot more on a child who is brighter, because I figure he or she already has a significant advantage over a child with less intelligence in terms of being able to get scholarships, select higher-paying occupations, etc. That said, if my superbright child wanted to go to Harvard and my less bright child wanted to go to the community college (or vice versa), I don't think things have to be equal as long as everybody is happy. Happiness, health, and fit matter to me.
And sometimes equal is unfair. In my family, my parents gave the exact same dollar amount to all four of us, which ended up not being "fair" because one of my sisters was smart enough to get a full-ride (including room and board) scholarship and graduated with all that money in the bank. It wasn't that she was harder working--she was just smarter. And one of my sisters struggled more in school, and I went to bat for her with my parents to give her more money than they gave me because she wasn't capable of working 20-30 hours a week and keeping her grades up like I was. It seemed fair that she get more support. So I think you really have to look at the situation.
One other word of caution, I have seen multiple families devote more resources to a more talented child, to the detriment of other children in the family, and I would strongly caution against it. One friend spends every single weekend at the rink, every evening at practice, and thousands of dollars every year on their oldest. Neither of the other two children have that opportunity, because the time and money is already allocated to the oldest child, and they are relegated to being rink-side.
Also, though it's easy to say it's not important to be "equal," to a child time and money are often seen as a proxy for love, and the child who gets more of them is viewed as being more loved.