Assuming I can find an equivalently desirable icon here, in font awesome, or an SVG; which should I prefer?
if you want something found on a CDN and as a result more likely cached in browsers (or pre-loaded via an extension), font awesome is a safe bet
If you want the most pure option, SVG (hint, change the background of the css icon boxes to see why, the usage of background-colored boxes everywhere is pretty damaging if you want to use it on anything other than flat colors)
if you want the icon to interact with your content (want text in a speech bubble? changing numbers? etc?) or to be quickly tailorable, then probably CSS icons