La Hacienda.
San Angel Inn had a period where the food quality REALLY fell off. It's come back (IMHO) quite a bit, so, really, both choices are good.
I've had the Mole (chicken at the Inn, Pork at La Hacienda) at both places, both relatively recently, and it was VERY good. I preferred La Hacienda, over all....but it was closer than it would have been even a year or two ago.