I would say night for two reasons:
Night time better matches the storyline and it is just easier to get into the spooky mindset.
Night time offers what I feel are the best views of DL when at the top of ToT, I love the way DL looks from that angle, you can see everything!
Another great time to enjoy the view of DL from the top of ToT is at dusk, very beauitful as the sun sets over the west.