"不应" vs. "不该": Which is used in formal writing? Recently I've been writing "should not" in Chinese a lot. But, I've been confused. What is the difference between "不该“ and "不应“? Are they both used? 

For example, I know we can say

对不起，我不该说那么难听的话。

but we could also say 

学习不应再是少数人的高级消遣

(Both examples from Iciba) My intuition says 不应  is more formal. Is that the only difference? Are there different usages?
Generally speaking, "不应" is used more frequently in formal writing, while "不该" is used in oral expression. 

If it's hard for you to determine formal or non-formal, consider this, use 不该 for subjective, and use 不应 for objective case.