Specifying model relationship as string vs concrete model?

Since Django provides two ways of specifying model relationship, 

models.ForeignKey('User') vs models.ForeignKey(User)

Which one is more preferred and recommended? Is there any upside or downside of choosing one over the other?  In Django documentation, all the example are of the second form but I have seen many people specifying the relationship as a string. Can Someone please elaborate more on this?

Regards,

Mohit

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