What is the grammatical difference between べきは and べきことは

Question

I usually see べき nominalized as べきは. Is it grammatically correct or just a casual way of saying? How is it different form べきことは?

Answer 1

べきは is a more pompous, not casual, way of saying べきことは.

• 今やるべきことは待つことだ。
  今やるべきは待つことだ。
  What we should now is wait.

べき is the attributive (連体) form of べし, and using it like a noun is basically an old way of speaking Japanese, although this is found in many patterns of modern Japanese:

• Plain Verb followed by がよい
• How can （の）目指すは be grammatical?

That said, べき is already a stilted word, so the difference between べきは and べきことは is not very large.