A lot will depend on your interests and the time you have, but when we went last summer, we did the evening tour of the Nasrid Palaces and then went back for General Admission the next morning.
The evening tour has a very different feel to it, more exotic, more focus on the reflections off the water and the sounds of the fountains. To me it was easier to see it as a residence and less as a tourist attraction. But in the day, you get better views, more ability to see the details, time to explore at your own pace, and of course seeing and walking in the gardens!
The Alhambra (and the Nasrid Palace, in particular) is amazing no matter what you do, but if you're able, my suggestion would be to do both!
Forgot to add: In the evening, some parts of the Nasrid Palace are closed that are open in the day--you'll still see the more well known parts, but if getting to see all of everything is important to you and you can only go once, you'll need to factor that in.