Scrivener exists, in part, to put manuscripts into standard manuscript format. The # mark it uses as the default section break is, frankly, a little puzzling to me. (Apparently this is standard for SFWA format, see the comments.)
Manuscripts in any variant of standard manuscript format will separate sections by either an empty line or three asterisks. However, when these fall at the end or the beginning of a page, they're easy to miss, so writers will often use lines of asterisks or similar symbols. Scrivener's default pound sign is fine for this purpose.
Leave the symbols and lines to the graphic designers. You're a writer, and you should worry about the words, not the design. Using lines of dashes in the manuscript, particularly when they're of random length, comes across to me as, frankly, slightly amateurish. But whatever you use to separate sections should be consistent throughout the manuscript.
Re-reading this question, I'm guessing that, since you're concerned with readers, this may be a situation such as an e-book or web publication, where manuscript format is irrelevant. If that's the case, it doesn't matter what you use, but I do suggest picking a simple marker such as three asterisks or bullets and centering them in the space in-between sections. With web publications and ebooks, you have no way of knowing how the text will hyphenate, so simply using white space in-between sections may not be the clearest way of breaking up sections.