The best solution will depend on factors such as the browsers in use by your user base, the knowledge any other people working on the same code base may have.

The flex box model is not supported at all in versions of Internet Explorer prior to 10, nor in versions of Opera prior to 12.1. It is incompletely supported in all versions of Internet Explorer, and in old versions of Firefox, Chrome, and Safari. However, all recent versions of popular browsers like Firefox, Chrome, Edge and Safari support it fully. The vast majority of Internet users today are using browsers which support the flex box. If you decide to use the flex box model, set the "justify-content: center" property, as well as ensuring your flex box has the "display: flex" property, to centre horizontally. To centre vertically, use "align-items: center" instead of justify-content.

On the other hand, centering by using margin is very widely supported, and more web developers are familiar with the technique, which could make your HTML easier for others to maintain.
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