I've just tried experimenting and although my experiments were perhaps simplistic, I could not detect any difference in the three cases so long as there was no space between the item and the marginpar.
My experiment was the following code:
\documentclass{article}
\newlength\shove
\setlength{\shove}{23.57pt}
\begin{document}
How much is that doggie in the window?\hspace{\shove}
The one with the waggly tail?
How much is that doggie in the window?
I do hope that doggie's for sale!
How much is that doggie in the window?\hspace{\shove}
The one with the waggly tail?\marginpar{That's ``tail'' not ``tale''}
How much is that doggie in the window?
I do hope that doggie's for sale!
How much is that doggie in the window?\hspace{\shove}
The one with the waggly \marginpar{That's ``tail'' not ``tale''}tail?
How much is that doggie in the window?
I do hope that doggie's for sale!
How much is that doggie in the window?\hspace{\shove}
The one with the waggly tail\marginpar{That's ``tail'' not ``tale''}?
How much is that doggie in the window?
I do hope that doggie's for sale!
\end{document}
By trial-and-error, \shove
was the maximum space that could be added there without shoving the "tail?" on to the next line. All three looked exactly the same in terms of positioning and spacing.
(Of course, there's probably an argument as to where to put it for semantic reasons, but I'll leave that to someone else as I don't know of a hard-and-fast rule to apply in this case so my opinion would be just that: an opinion.)