I've stayed in both the deluxe and nondeluxe and do not feel the extra 4 feet of space is worth the difference in price. The deluxe rooms are on the 5th and 6th floors (6th floor is concierge level) so you get a more expansive view. You can have a 5th person in a deluxe room; the nondeluxe rooms are limited to 4 people (unless the 5th is an infant).
Sofas/day beds aren't found only in the deluxe rooms. Our last trip we had a room with a king bed and a day bed on the 3rd floor overlooking the Arusha savannah. We didn't really use the sofa at all. If we wanted to sit and relax, we went out on the balcony and watched the animals!
Being on the 5th floor might be a good thing with a pool view room but with a savannah view room, I prefer to be on a lower floor. I've been in both 5th floor and 3rd floor savannah view rooms and preferred being closer to the animals.