Over the years, the Inn (remember, they're both "San Angel", which is the company that runs them both) had gotten a lot of comments about their food being, shall we say, "not particularly memorable".
However, when La Hacienda opened, I saw a definite improvement in the food next door at the Inn. I had lunch there just a few days after La Hacienda had opened, and of the half dozen or so times I've eaten there over the past 20 years, this one was definitely the best.
I also liked my meal at La Hacienda. I think it was the second or third day they were open, and they were working really hard to get things right - had two or three managers talk to me during my meal.
The way they explained the difference to me was that the two restaurants represent food from two different parts of Mexico. I think La Hacienda was the more southerly part of the country.