While the term 'left-wing" is relatively neutral, "leftist" has a derogatory air to it, or so it seems to me. Am I correct? Does anyone have any references for this? All I could find was this, which suggests that the derogatory nature associated with the word is more common in the US.
If I say you have left-wing leanings, I am saying you fit in that camp because your beliefs are progressive. If I say you are a leftist, that is an attack on you as a person, the whole you, so it is more personal.
If I think you have a sick idea, we can disagree and debate it. If I say you are sick, those are fighting words.