What's the difference? Are they really different, with a nuance, or is it just a matter of preference? Does one point to your level of fluency more than the other?
Made up examples.
"I just want to say that for all the experience she claims she's had, she is not all that competent."
"I want to just say that for all..."
Would the position of just set the tone for what the speaker is about to say? In "I just want to say..." the speaker might be about to make a comment directed at another comment made earlier or someone and perhaps oppose it or critique it . In "I want to just say..." on the other hand, the speaker may be wanting to add a piece of information, which would keep the tone more neutral.