I saw on the Vault's NV bugs list that if I tell Hardin about the tapes to oust MacNamara before finishing the filtration part of the quest then it won't work.

But then somewhere else on the page it said it would work if you've gotten the objects for fixing the air filter, even if you haven't done it.

So which one is true?

The first one is true because I don't see the second one anywhere on the page. Gathering turning in the air filtration parts and ousting MacNamara are meant to be exclusive of each other.

In any event your better off keeping MacNamara in charge, there is no real advantage to putting Hardin in charge other then getting to skip gathering the air filtration components. Their are however some negative consequences and you may be back here complaining about them should you put Hardin in command.

Agreed, Leaving him there is the best thing to do. Sometimes the Optional quest choices are bad. Dunno what would happen if you did get him fired.