I love Animal Kingdom at night. Now, if you could only visit either during the day, or during the night.... then day would be the way to go. But for a different experience, I wouldn't skip out on a nighttime visit.
Pandora takes on a totally different feel. The safari after dark is a cool experience, although you probably won't be able to see many animals. Tree Of Life Awakenings is worth stopping for on the way in/out.
Downside, is that you pretty much can't see any animals after sunset, besides the few you may see on the safari. So, if you would be spending less time at Animal Kingdom during the day, since you'd be there at night aswell, then you might want to skip nighttime if animals are a priority for you.
P.S. If you can be there during sunset, Definitely try to time a ride on Kilimanjaro Safari for just before sunset. I've always seen more animals before/during sunset than any other timeframe on the ride!