\emph or \textit

Those are very different commands even if the output happens to look the same.

• If you want to emphasize a word or some text, use \emph. Don't just make the text italic or bold. If needed, you may change the behaviour of \emph whenever you wish in the preamble and the whole document will be adjusted accordingly.

• If you want to get italic text, use \textit. \emph might have a different effect, a package like ulem might change it to underlining for instance.

• \emph may be nested: emphasized text within emphasized text may be upright. In contrary, nesting \textit just keeps the italic shape.

• Further, I rarely use physical font commands in my body text. I use them to define styles in the preamble and use the styles in the document afterwards, ensuring consistency and allowing changes to be easily made.

The \emph macro is designed to be semantic markup. So while by convention it makes text italic, this is not always the case. For example, the beamer class makes \emph text red as this works better in presentations than using italic.

On the other hand, \textit makes text italic, with no variation. Thus it is intended for making text italic when that is exactly what you want.

I would usually favour using \textit in a design context, where you are deciding 'these things should be italic'. Normally, that would be in the preamble or a class file, perhaps something like:

\newcommand*\latin[1]{\textit{#1}}

In the document body, I'd favour the semantic approach and use \emph, as this can then be defined in the preamble to do something else entirely.