Ignoring the fact that threads like this are completely useless, I'd go with wet weather.
Tbh, I think night races would be a try-once-and-then-never-again gimmick, and I can't see the appeal of driving fast without seeing the track. Most people can't even handle the effects of wind, how do you think night races compare to that? (Though granted, that is an argument valid for both night and wet races.)
The only time where night races (implying a dynamic day-night cycle) would be cool is on the 24h race, but that's about it. Driving in rain, however, is something much more common to happen in real racing. Having to deal with changing conditions and pit strategy sounds, at least to me, much more interesting than setting the monitor brightness to max.