Title case works for bringing attention to specific parts of a text, like titles and headings. Sentence case is easier to read in most cases, and is always best to use for anything longer than five or six words.
Title Case
With title case, the first letter of each word is capitalized, except for articles and prepositions. This is how book titles work, for example. Always use title case for:
page/post titles (Heading 1 style headings)
page headings (Heading 2 style headings within text)
document titles
book and other media titles
references to other websites
proper names, including product names
drop-down menu names
form titles
Event titles
Sentence Case
With sentence case, only the first word and any proper nouns are capitalized, as in a regular sentence. Use sentence case for:
subheadings (within a page, these might be heading 3, 4, 5 etc. style headings)
navigation
buttons
any other content not listed in the title case list
checkboxes and radio buttons
menu items
form fields