Eiffel Tower - Day or Night?

We like it early - get there before it opens (at 9:00am, I think) and get in the line. You're up when the breezes are fresh and there may be slightly fewer people. You can take the stairs - pretty cheap, but you need to be in shape and not scared of looking down through the steps. The 2nd level is cheaper than the top. I am a height junkie, so the top o' the tower is best for me. If you're not crazy about heights, you might not like it much. At night, I imagine it is very romantic to look out over the lights of the city (you mention "as a couple" - so I am guessing romance might be a consideration) but honestly, we prefer watching the light show on the Tower at night. Cross the river to Trocadero and watch from the steps. The fountain splashes atmospherically, the lights are gorgeous, and it is a wonderful place to sneak a little romantic snog. :D

It is similar to the Empire State Building, if a clear day you can see all over the city, if overcast won't see much. We were there several times and did both. I prefer day time and then we went back at dusk with some wine and cheese and watched the lights go on and had a little picnic from the nearby park. Either way, is magical.

Rob,
I've been up the tower twice - once during the middle of the day, and the second time at dusk. Based on my own experiences, being on the tower at twilight was a much better experience. I pre-booked our entry based on sunset, which you can find easily for your particular date on the internet. I wanted to be on the second level at sunset so we entered about an hour before riding the elevator. We were on the tower about 90 minutes total, which the last 20 minutes or so spent descending by stairs. We walked off the last steps right at 10:00 pm when the tower's light show began, so we could step out on to the Champ du Mars and watch it from ground level. YMMV. Good luck and happy travels.

I like daytime (early morning has fewer crowds, but too early and you face morning haze). I prefer the view from the Eiffel Tower during the day, but the view of the Eiffel Tower in the evening/night.

We were there in the fall of 2009. WE went up during the day because that worked in the schedule. We went back at night. At 9:00 and 10:00 the tower twinkled. I don't know if it still does or not. That was nice.

Both! Someone recommended to me that you go up close to dusk and do all 3 levels. Go back to level 1 and hit one of the cafes until it gets dark. Then go back up to level 2. When you are done, catch one of the cruise boats at the base for a night cruise on the Seine.

Both!!! They are equally as good so go up at both times.