Hi,

I have difficulty using the content of subject phrases when they seem to mean basically the same thing in sentences. I think 'sort/kind of' is used to ameliorate the effect or make the subject matter trifle, as if it is not so much important; whereas, 'a kind of' is used when making distinctions, in this case being one kind of the noun.

But sometimes, its not so clear because I feel both can work to produce similar effect, especially when the phrases are used before a noun.

sort/kind of a done deal

a sort/a kind of done deal  

The above examples might not be a good one to go by but those are what I can come up with.