1/2 words? Title case, as a command.
3+ words? Use sentence case as if you are carrying on a conversation.
Sidenote: Adding certain words like “now”, “today”, “instant” or “quick” adds a perception of urgency for the user, but you already knew that.
Stolen from:
http://uxmovement.com/buttons/best-practices-for-call-to-action-buttons/