'It's my ....' means 'This is mine'. Often we'd say this in response to the question 'Whose ... is this?' or 'Is this your ... or your brother's?'. It means that this thing belongs to me, rather than to someone else. 'It's a .... of mine' means 'I have several ....s, and this is one of them.' They are often interchangeable, but the focus is slightly different. Which one you choose would depend on the context.

With the first, there is an implication that you have more habits, whereas the second, perhaps it is your only one. But mainly, I think that it is what feels natural - I would only say the first.