I recently did a lot of testing trying to get uniform spaces between the ellipses and double/single quotes and trying various set widths. What I discovered is that my Sony reader will have a uniform space of its own choosing when I used between the periods of the ellipse no matter what width you try to set in a stylesheet. The ellipse is never stretched out to justify the text, and using makes it never fall on another line, it stays with the previous text, whether there's 3 or 4 periods for the ellipse at the end of a sentence.
When viewing the same epub through Sigil's or Calibre's viewers, the spacing will be uniform only if the text is not justified. If it's justified, then the PC viewers will stretch out the spacing between the periods as it needs, making it look pretty bad horrible. It ignores any character width that you set in the stylesheet. If I made an epub strictly for PC or tablet viewing, I'd probably switch to using the ellipse character only no matter how the original book had it.
So I'm happy with using for the ellipse since I only read from my ereader. I'm assuming most ereaders follow the same rules as the Sony, but really don't know for sure, would be nice to be able to confirm that.