I'm gonna predict a Utd win after the ref lets them get away with murder - the kind of game where Utd will tackle heavily, the ref will let it go for half an hour, then after he finally books one of their players he immediately books an Arsenal player for some half-offence to "even things up".
Add in a penalty and red card not given and we're golden - see Jagielka from Everton and Henry from QPR.