Are you fast or slow eaters? Are you fast or slow walkers? The Land is pretty close to WS, so it won't take you very long to get there after dinner, but that's probably not going to be the make or break issue.
As the PP indicated, there is always the possibility that the restaurant is running behind.
Overall, you need to ask yourself, which is more important: eating at Garden Grill during a 'normal' dinner time or seeing IllumiNations? If the answer is seeing IllumiNations, then go with the earlier seating. That way you won't spend the entire meal looking at your watch worrying about the time.
My guess says that with seven people and a little under two hours for dinner, you'll probably make it out just about the time IllumiNations start (assuming you eat fairly quickly). You'd be able to see it, but not 'stake out' a spot or be at the front near the railing.