Is heatsink the higher the better

Not necessarily. Theoretically the best one is to have the biggest heat dissipation surface with biggest air flow advantage. However, confined to case height and space limit, we can't always get what we want and we must make our choices. It's better to conduct real test and get a result. There are three combinations:

a. 10mm heatsink + 50x25(H) mm fan
b. 25mm heatsink + 50x10(H) mm fan
c. 28mm heatsink + 50x7(H) mm fan

Above all three options, the best goes to (b), lower cost and better cooling effect