First of all, a visit to the Eiffel Tower must be planned; otherwise you could end up wasting loads of precious time in really long queues. The tower has a very informative page that also gives people the option of buying tickets in advance, which is just perfect.
Link here: http://www.eiffel-tower.com/en/preparing-your-visit/planning-your-visit.html
Another recommendation I’d give you is that you have to be really cautious with the time on the ticket, it’s only valid on the hour, and otherwise it will be subject to availability.(Believe me, it gets crazy packed sometimes) It happened to me, I was lucky enough that when I got there they let me in, but I ran across every street of Paris, on every train, on every ferry, Just to make it on time; but I wasn’t.
Now, if you need to decide what’s the best time to go up the tower, day or night? I’d say sunset. The best from both worlds, beautiful for pictures, beautiful for the soul, beautiful for your couple, beautiful for anyone; check the sunset time and buy the tickets online. (I was supposed to get there by sunset, but as you can see my pictures I didn’t make it) There is something else that needs to be planned.
Going to the top of the tower or just the observatory in the middle? My answer is: it depends. (Traveling is not easy, too many decisions to make for such short time) I would say yes, go to the top but take pictures from the second floor.
If you go there for sunset, plan to see the sunset on the second floor. Go to the top first and then head down to the second floor just before the sunset.
Incredible view of Paris, and also an incredible experience on this beautiful structure; the mother of Paris, the one that spends night and days vigilant, paying attention to everything that happens in the city of light.