Depends on its Role
This seems fairly obvious, but based on the rest of your team, both the speedy but frail mid-day form and the somewhat hardier Midnight form play slightly different roles.
According to the latest stats I could find,
120 | Lycanroc | 0.85165% | 24973 | 1.171% | 19570 | 1.236%
221 | Lycanroc-Midnight | 0.21727% | 9764 | 0.458% | 6961 | 0.440%
While I might be wrong, and there are other usages (like another part showed Lycanroc at 113th place with 1.23% and Midnight at 193rd spot with 0.44% usages. This, I think was from all OU games as opposed to previous stats which I think are for higher rated - above 1800s - battles).
Either way, according to the people, Lycanroc is more popular than the midnight form. And this can be perfectly reasoned out:
Most people use it as a Physical attacker. Great Attack and Speed stats mean it makes for a great Banded or Scarfed Set, or even Life Orb. It can even set up Stealth Rock, and here's something I personally feel is responsible, but can't prove, is the Midday Forme's ability to use Accelerock, a priority Rock Type move! In the meta game, Priority plays a crucial role, and although Midday form is fast all by itself, you can never be too sure about needing Priority.
The Midnight forms suffers from a lack of an excellent moveset, getting moves like Counter rather than Accelerock. Further, Rock is a terrible type for defensive Pokemon. With nearly every Pokemon carrying moves like Earthquake, Scald, Iron Head, Giga Drain, Close Combat whenever they can (specially EQ and CC), even the defense buffs do not help it out much. Further, its function is also to Stealth Rock, and KO certain Pokemon. The only way to make it a Physical Sweeper (which is the most preferred, specially by newer users; experienced users tend to ignore it altogether) is to Scarf it, which, while a decent idea on paper, is sometimes not worth the risk of getting locked into the same move, specially for a defense oriented Pokemon.
This does not particularly mean Midnight is bad, just, to answer your question, Midday is better. Of course, this is a generalized version of the answer, and player opinion changes based on play style and usage, the bottom line is, get another Pokemon to set up Stealth Rock and focus on defense and use the Midday form to sweep.
