From the OED, complete with the weird Oxford spelling:
Patronizing: "That patronizes a person or thing, esp. with an air or assumption of superiority"
Condescending: "That condescends; characterized by, or showing, condescension. Now, usually, Making a show, or assuming the air, of condescension"
They both require an air of superiority, so it is difficult to take the degree of 'air of superiority' as the difference. Take a look at the verbs:
Patronize:"to act as a patron towards, to extend patronage to (a person, cause, etc.); to protect, support, favour, or encourage."
Condescend:"To come or bend down, so far as a particular action is concerned, from one's position of dignity or pride; to stoop voluntarily and graciously; to deign"
This is where the difference lies. If you tell the other guy his new 1,000cc hatchback is wonderful, and you nearly bought one yourself, while you have a Porsche parked in your garage - you are being condescending.
If you tell the other guy that you will help him to earn more money so he can buy a 2,000cc saloon car, because you know how difficult it is for him to get promoted, you are being patronising.