Difficult decision...Another factor you need to add is distance; it's not like LC and FEA are right next to each other - I'd allow AT LEAST 10 min to get from one to the other. Is it possible? Yes, as long as you are seated on time and the service is efficient (personal experience is yes to both). The good news is that you can be up to 15 min late for an FPP, so that will give you a bit of a buffer.
I NEVER try to time a dinner reservation with IllumiNations. I always plan on eating first and then finding a spot to watch. LH has very few "window seats" and you can be sure that most of the people geting there at that time are trying to do the same thing you are. But unless LH is running way behind, 90 should be plenty of time to eat and exit before the show (and the area between Mexico and Norway is one of my favorite places to watch).
Another factor is the type of dining experience you're looking for, and the two you've picked are quite different. LC is a signature, which means it's a "high end" steak house; excellent steaks, but at a price. LH has quite good Mexican food at a markedly lower price. I love signature dining and probably do a lot more of it than most people do on my trips, but when I'm paying $50-75 per person for a "fine dining" meal, I want to enjoy it as an "event" unto itself and not have to rush through it to get somewhere else at a specific time. With these data points, were I in your shoes, I would go with LH.