Both are used interchangeably in both properties, but "Druchii" seem to be used more as a derogative (in 40k you generally only see craftworlders using the term) while "Drukhari" seems to be a more generic term as [Thing]-hari and [Thing]-yani seem to be the typical romanization of Eldar terms for cultural groups. (we see less of the Elves in this area, ironically, since they have more sharply defined cultural groups that are more akin to ethnostates than the primarily philosophical differences between the Craftworlders, Exodites, Corsairs and Dark Eldar.)
