Do both!
The land really cannot be fully appreciated until you see it in BOTH daylight and at night time.
It will be a completely different experience visually and atmospherically between the two time periods...as different as, well, 'day and night'...!
I recommend waiting to do the 'Na'vi River Journey' until night time, only because I find that doing dark ride experiences in the evening enables you to better see the interior elements.
Your eyes are already adjusted to the lower light levels, so it just works.
'Flight Of Passage' will be a must do, but can be done in the daytime or evening...whichever works for you.
Both the ride experiences and the land are going to be terrific and full of some beautiful eye candy.
It pays to pay attention to the details, and Pandora will be packed full of them.
The recent photos and video clips are just scratching the surface.
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