Which one to use
Sentence case is the safer default. It reads faster, it survives translation, and it does not force a decision about every preposition. Google's developer documentation style guide uses sentence case for headings and titles, and most product teams have followed. Apple, Microsoft and a good deal of editorial writing still use title case for headlines.
Title case has no single definition. AP style keeps articles, coordinating conjunctions and short prepositions lowercase, and capitalises the first and last word plus verbs, nouns, pronouns, adjectives and adverbs. APA, Chicago and MLA differ on the details, mostly on how long a preposition can be before it earns a capital. Pick one guide, write it down, apply it to everything.
Capitalise Each Word is a third thing. It raises every word with no exceptions, giving The Art Of War. That is what the macOS Edit menu offers under Transformations, alongside upper and lower. It is not title case, and it looks wrong in a headline.
The case chips behave as one exclusive group, so flipping from Title to Sentence needs no reset in between. Each transform runs against the original clipboard text rather than against the previous result, which is why switching back and forth never compounds.
The same panel carries the affix chips. Numbering every line, or prefixing each with > , stacks on top of a case change in the same paste.
Switching a headline without retyping it
On a Mac, in any app that accepts a paste:
1. Copy the headline.
2. Press ⌥⌘V to open the paste preview.
3. Tap Sentence case or Title Case.
4. Press ⌘V.
The macOS route is narrower. Edit and Transformations gives you upper, lower and capitalise, only in apps that expose that submenu, and there is no system-wide shortcut for it.
The trap
No tool applies your style guide. Klipto's Title Case follows simple word rules and has never read AP, Chicago or your internal handbook. It gets the common cases right and leaves the judgement calls to you. Any tool claiming full AP compliance is worth checking against a real headline with a four-letter preposition in it.
Acronyms and proper nouns break on the way down. Converting a title-cased headline to sentence case lowercases words a machine cannot tell are names. The Rise of iPhone in Asia needs a human eye afterwards. Short headlines are fine. Long ones with brand names need reading.
What I use
For one headline, use a web page. Try it free in your browser, which also covers camelCase, snake_case, kebab-case and the rest.
I built the chips into the paste because I was changing case forty times a day and a browser tab was the slowest part of it. Klipto puts eleven styles in the paste preview, offline, in whatever app I am working in. If you only need upper and lower, the macOS Edit menu already has you covered.