Actually, that's a very good question and honestly I think it comes down to what you think would interest you more.
Since I never sleep well the first night I ever stay anywhere, I went on EMH evening on the night we arrived and really enjoyed the experience. I basically spent 2+ hours exploring and taking photos (using a tripod). I also got to walk right on Na'vi River Journey at about 12:40am.
If I was able to redo it though, I think I would have wanted to go during the day first. Since my primary reason of going at night was to shoot photos, It would have allowed me to get a better lay of the land. Like others have stated, it is fairly had to see at night and While I was happy with my shots, I would have been able to take more, if I had known where things were.