K.P.
I would do more for the college grad, but my observation is that the standard gift for HS is around $50 and college $100 (if you can afford it). You could do a gift card for the HS grad who is going off to college too without have to spend quite as much!
It's not favorites, it's level of accomplishment and intensity. Graduating from college is a major lifetime accomplishment. When the son graduates from college, you'll give him the big $$!