The Montjuich term is supposed to be the original medieval catalan term, so it would be correct to call it that way.
My two grains of salt, as neighbour from Portugal who follows Spain/Catalunya from relatively close: I would venture in saying that Montjuich was the spelling under Franco (who tried to squash the use of Catalan) and, possibly, already under democracy, when Catalan was revived and the spelling Montjuïc was established as the correct modern Catalan form, the Spanish version was changed to Montjuic.
