Which of the plaintexts is more likely to be the original plaintext and why?
A very literal and devious answer to the question (at least as you have posed it) is that 'ATTACK AT DAWN' is more likely to be the original plaintext because it can be independently established that attacks at dawn are more common than attacks at noon. 😑
Which means there's a subtle flaw in the question: what cryptanalysis is supposed to accomplish is to use the ciphertext as evidence that favors some candidate plaintexts over others. The point isn't really which plaintext is more likely, it's whether an analysis of the ciphertext can rationally demand that you change your mind about their relative likelihood. Like, you go into it thinking that 'ATTACK AT DAWN' is twice as likely as 'ATTACK AT NOON', but analysis of the ciphertext compels you to conclude some other proportion.
The point of all this is the often-missed nuance that the famous perfect secrecy of one-time pads is not that the attacker cannot figure out what candidate plaintext is more likely, but rather that analyzing the ciphertext is of no use in achieving that end.