What are the pros and cons of players knowing each others' secret backstories?

Question

First time D&D 5e GM here.

At the beginning of my campaign, an NPC will try to send the party on a covert operation. I'm giving one of the characters a special role and backstory: He's that NPC's "mole", and knows special details about the mission, which he is supposed to spill when the time is right.

I'm ok with the characters finding out "too early" about those details (I don't want to railroad), especially if the mole behaves in an unusual way, makes seemingly contradictory decisions or pulls secret knowledge out of his hat. But I'm seeking advice whether the other players can know about his backstory beforehand? What are the most important points in favour or against doing so?

Some thoughts I have had so far, although I lack experience to weight them:

Pro

• The players really enjoy learning about each others' characters' backstories.
• It might be hard for the player to keep his secret all the time.

Con

• It takes an interesting surprise out of the story
• It's hard for the other players not to meta-game, i.e. to let their characters not know what the players know.

(Related question: How to discuss information only my character knows with the DM during the game?)

Answer 1

Since you say you're a first-time DM, I might exceed the scope of your question slightly in my answer. You're up against a philosophical question: Are the players co-tellers of the story, or "passengers" in it? This affects whether or not a particular quality is a Pro or a Con.

Players as Co-Tellers

In this approach, you treat the players as storytellers, and your job as DM is to arbitrate disputes between them (like an editor working with multiple writers) and to provide a general framework for this to occur. You keep the 'grand vision' of the world, while they cooperatively tell stories within it.

• Pros of revealing backstory in this approach: The players are able to incorporate one-another's stories into their narrative plans, and can decide how to use their own characters' motivations to further the 'mole' narrative.
• Cons of revealing backstory in this approach: Minor, but if the other players don't like the narrative, they'll be well-equipped to bypass it.

Players as "passengers"

In this approach, you treat the players as mostly reactionary inhabitants of the narrative, rather than near-equal participants in crafting it. This more closely resembles the 'traditional' DM/player relationship, with the players and characters both being surprised by in-game events. It allows more authentic reactions on the part of the players, but can sometimes create tensions since the DM decides how the characters' arcs will unfold rather than the players getting to develop their own visions.

• Pros of revealing backstory in this approach: There really aren't any. It basically forces the players into the co-teller role, which might make your job as DM easier but ignores the whole point of this approach.
• Cons of revealing backstory in this approach: It ruins the surprise and authenticity of the players' reactions, and essentially forces them out of their characters' heads.

Answer 2

While I feel this is more of an opinion oriented question, I will do my best to answer it. In short, no, they should not know the secret background of the character in question.

Here's my reasoning. As a Con you stated that it's hard to not Meta-game, and that's correct, so why give your game the risk of having it happen? Not to mention it's suppose to be secret. That said you could draw up or have the player draw up a different backstory they stick to until it's revealed, or they find out and force his hand.