I think the day map looks more interesting because you play a lot with the outside setting, and in the night, its way more hard to see it, but it can be a cool thing if you have a lot of problems to lead the eye of the player.
So for me -> day map for the look, but if you see that you have some problems with the visibility of the player, why not try a night map.
These heights scares me, be aware of the fact that in tf2, the height is very important, and these doors on the top seems really hard to reach, but the playtest will say more.
Also, I already wich you good luck to balance this map with all this height variation, did you already made some test with bots to see if it's not a steamroll for a team ? But, it's already a really good project, and has a level designer from BMS, i'm sure you will turn this one into a solid map.