They are both good, but the menus are different. You should look at both menus and see what dishes you and your family might prefer.
Some other thoughts:
La Hacienda de San Angel is the restaurant outside of the Mexican pavilion adjacent to the World Showcase Lagoon. Service here is a little bit better, and I'd probably give a slight edge to the food.
San Angel Inn is the restaurant inside the pavilion. The atmosphere is great. Service can be more hit and miss, and the tables are somewhat close together.
We have eaten at both over the past few years and like both of them.