В чем разница между словами against и versus ?

В происхождении.
versus - латынь
against - из старо-английского

versus (prep.) mid-15c., in legal case names, denoting action of one party against another, from Latin versus "turned toward or against," from past participle of vertere (frequentative versare) "to turn, turn back, be turned, convert, transform, translate, be changed," from PIE *wert- "to turn, wind," from root *wer- (3) "to turn, bend" (cognates: Old English -weard "toward," originally "turned toward," weorthan "to befall," wyrd "fate, destiny," literally "what befalls one;" Sanskrit vartate "turns round, rolls;" Avestan varet- "to turn;" Old Church Slavonic vrŭteti "to turn, roll," Russian vreteno "spindle, distaff;" Lithuanian ver čiu "to turn;" Greek rhatane "stirrer, ladle;" German werden, Old English weorðan "to become" (for sense, compare turn into); Welsh gwerthyd "spindle, distaff;" Old Irish frith "against").

against (adv.) early 12c., agenes "in opposition to," a southern variant of agen "again" (see again), with adverbial genitive. The parasitic -t turned up mid-14c. and was standard by early 16c., perhaps from influence of superlatives.

again (adv.) late Old English agan, from earlier ongean "toward, opposite, against, in exchange for," from on "on" (see on) + -gegn "against, toward," compounded for a sense of "lined up facing, opposite," and "in the opposite direction, returning." For -gegn, compare Old Norse gegn "straight, direct;" Danish igen "against;" Old Frisian jen, Old High German gegin, German gegen "against, toward," entgegen "against, in opposition to."

In Old English, eft was the main word for "again" (see eftsoons), but this often was strengthened by ongean, which became the principal word by 13c. Norse influence is responsible for the hard -g-. Differentiated from against 16c. in southern writers, again becoming an adverb only, and against taking over as preposition and conjunction, but again clung to all senses in northern and Scottish dialect (where against was not adopted).