Where should I place the negative?

The other day I watched a film in English (something I seldom do), and I was taken aback by a construction that I heard several times. The film was "The Sound of Music"; the example I remember best is when Maria and the Captain have just stopped dancing the Ländler. Maria is told that her face is all red, to which she answers "I don't suppose I'm used to dancing".

My question is not whether this is grammatically correct (it obviously is), but whether it's more common or better than "I suppose I'm not used to dancing" (which is what I would have said). Or are they completely synonymous? To me, there is a subtle difference in meaning between both sentences. In fact, the sentence in the film doesn't make much sense in this case. I mean, her being all red doesn't depend on her supposing or not supposing, but on her being used to dancing or not!

I don't know why, for some reason, it sounds right to me in some cases and not in others. To keep within the bounds of the film, I have no problem about "I really don't think you're the right man for me", since it sounds as good as " I really think you're not the right man for me". Maybe I'm influenced by Spanish, in which, of these examples, only the latter sounds completely natural in both constructions (but I don't know why!)

How would you say it?

Hi Philip,

Thank you very much for your answer. What you say sounds very sound (ahem!). I see what you mean, and how it sounds less curt in cases like my second and your second examples. I had thougth that perhaps the difference between both constructions might be a matter of meaning, but I see it's more a matter of intention, instead.

Philip wrote:

[I'm glad you mentioned Spanish:  "no supongo" sounds odd to me as well, and I'm not even a native speaker of your beautiful language.]

I'm glad you like it. You must have a very good level if you can tell that sentence sounds odd to you!