JM........Hope this helps.......
Both are great, but here's how I would look at it.......
If you just want to stop for a quick bite to eat, while touring the Showcase of Nations, then the Cantina is your place...........
If you are planning on having a relaxing, 'sit down' lunch or dinner and drinks, then you will want to go to the Inn.....
Keep in mind, you may have to make a PS if you are going to the San Angel Inn, depending on what time of year and what time of day you plan on going........Whereas, the Cantina is "walk up" counter service, so no PS's necessary or available, for that matter.......
I could not tell you if the food comes from the same kitchen, but considering they are on two different sides of the walkway (street), I'm going to guess that they have separate kitchens....