I do a fair amount of highlighting PDFs - both PDFs created electronically (and thus very "clean") and PDFs created by scanning books, etc. All I can say is that highlighting in Adobe Reader is quite awful. At least 75% of the time I try to highlight a line, a different line or a different word is selected and I have to do "undo". I have tried starting my cursor in towards the top, middle, bottom, of a line, and really close and not-so-close to a word. There's no logic to it. Reader just selects whatever it wants to, even if my cursor was nowhere near a certain line or word.
I have tried Reader X on Mac and Reader X on Windows. Same thing - no difference.