When would you use each? I understand "condescend" has another meaning of "voluntarily lower oneself" (Babylon definition). But in the "look upon other" meaning, which would you prefer in which situation?
There is something of a circular definition at work here: to patronise someone is to behave condescendingly toward them, and to condescend is to behave in an arrogant manner by patronising those whom you consider inferior. I'd say they're fully denotationally equivalent. Connotation-wise, however, there is a slight difference: "patronising" implies behaving as a parent does toward a child, while I think "condescending" is more general.