better during the day or at night?
All I can tell you is we did it at dusk, which was really nice. The light was very nice and made for good pictures. Have not done earlier in the day or at night.
We rode the eye at dusk. The light was great and it was a beautiful ride. The ride takes about 30 minutes so we booked a time and got in line and timed it just about right to be 1/4 way into the loop just as the sun was setting. This gave some beautiful color on Parliament, Clock Tower, and Westminster Abby.
We went at dusk! Such a great view. We were able to get some pretty good pictures on our flight.