Saruman for or against Sauron?

[Kuruharan]

If they had not why would Sauron, who knew the nature of the Ring better than any, expend so much effort in trying to retrieve it?

Why does anybody do anything that would make things easier for them or get them more information? My guess would be is that Sauron was not sure what would happen if somebody else attempted to claim the Ring.

This is not at all surprising considering that Sauron himself did not seem to be aware of all the aspects and potentialities of the Ring (see his belief that it had been destroyed...)

Do we believe Tolkien or Gandalf?

Perhaps The Man himself can answer for us in a passage that could illuminate this discussion where he says regarding Treebeard's knowledge of the creation of orcs...

Treebeard is a character in my story not me
-Letter 153

There is clear differentiation here between what Tolkien knows and what his characters know. You say that Gandalf implied that Aragorn was capable of wielding the Ring. Tolkien explicitly denied that Aragorn would have been able to do so successfully and so defeat Sauron. Aragorn, for all his high heritage and nobility of character was not on Sauron's level.

I don't think there is any doubt about which opinion we need to follow in this case.