La Hacienda has only indoor seating. The patio area is actually the seating for La Cantina, the quick service in the Mexico Pavilion.
There are some window tables at La Hacienda that have a view of IllumiNations, but there's no guarantee that you'll be seated at one if you keep the 7:45 or 8:15 reservation. You can ask when you check in, but it is only a request and might not be allowed, even if you offer to wait for a window table. Some posters have said that La Hacienda does not accept requests and will seat at the next available table for your party size. Also, most likely anyone with a window table at either time will stay until after IllumiNations so at best you can possibly be seated at a table with a partial view or worst case - no view at all.
Neither 7:45 or 8:15 is likely to give you enough time to finish eating and find a good spot around the lagoon for IllumiNations. If seeing the fireworks is most important keep the 5:15 reservation. If you are going for the food and IllumiNations is secondary, choose the time you like best.