I had a situation where I needed two 'if' statements that could both go true and an 'else' or default if neither were true, not sure if this is an improvement on Jossef's answer but it seemed cleaner to me:
ng-class="{'class-one' : value.one , 'class-two' : value.two}" class="else-class"
Where value.one and value.two are true, they take precedent over the .else-class