The day visit includes all parts of the Alhambra.
The night visit only includes the Nasrid Palaces
Most people would go on the day visit and go on the night visit as an optional extra.
The night visit has a very quiet relaxed atmosphere. It is different from the day visit.
It is all light up by lights which gives a different perspective.
The sound of water dripping and flowing seems stronger in the night visit.
If you really like the Alhambra them maybe it is good to do both.
Many would say that the day trip is a must-do whereas the night visit is "well worth doing"
This is a good page about the Alhambrahttp://granadainfo.com/alhambra/