We were up in the air about what resort to stay in as well and one of the deciding factors was the size of the beds. DH and I are used to sleeping in a king sized bed, so anything smaller than a queen just won't work. None of the moderates have queen beds. Believe me, I already checked. So it's deluxe for us.
Also, your MIL may be planning on enjoying the resort more than you are. I know that by the 3rd day, my mom would be wanting a little time off. The moderates and deluxes offer more opportunities for such relaxation without having to stay in the room.
I'd go for at least the moderate.