Both text effects are used for valuable English vocab items.

🙂 double-slash is usable in plain text
🙂 italics is the better choice for longer strings, esp. when you don’t want the highlight due to bold or color.

— visibility/highlight .. For /one to three/ words (but not beyond), double-slash can sometimes be more visible than italics.

Without bold or color, italics alone is not really effective for highlighting/visibility.

_Incidentally_, double-underscore is more visible than /xxx/ or italics.