无敌 means 'invincible, unmatched' and is used as a predicate (e.g., 天下无敌 'unmatched in the world'). 无比 also means 'unparalleled' but is more often an adverbial/adjectival modifier (e.g., 无比强大 'incomparably strong'), while 无敌 implies being unbeatable in a contest or field.