What's the difference between Mi and Mi Note model lines? I assumed that the Mi Note was created to compete with the Samsung Note series. My primary use case is using a stylus and stylus related software.

But I can't find any reference to a stylus for the Mi Note(or the Mi Note Pro) model lines.

Is the major difference just screen size i.e. the Mi Note is a phablet?
Unfortunately yes - none of the established "Notes" from Chinese manufacturers (e.g. MI/Redmi Note, Meizu M* Note) actually has a pen or supports one.

It's just that most of us Chinese (not including me) seem to like large screen phones, and prior to MI Note, the flagship MI 4 could not fill into that blank. The "Note" designation is to distinguish the large screen variants, and in a sense, to give corresponding models a more "high-end" position - MI/Redmi "Notes" are indeed of higher spec and arguably better crafted than their non-"Note" counterparts at the time.

If you want a pen on the cheap, just grab an older Note model from Samsung, and optionally flash a ported firmware if you want features from newer models.