Yes, they are
almost identical, but I do see some differences in the content.opf file.
for one thing, Sigil ver 0.4 uses a different order for the parameters inside each metadata item. for example, version 0.3.4 has:
Code:
< <meta name="cover" content="cover.jpg"/>
< <meta name="Sigil version" content="0.3.4"/>
...
< <item id="ncx" href="toc.ncx" media-type="application/x-dtbncx+xml"/>
< <item id="cover.jpg" href="Images/cover.jpg" media-type="image/jpeg"/>
< <item id="style001.css" href="Styles/style001.css" media-type="text/css"/>
< <item id="content0001.xhtml" href="Text/content0001.xhtml" media-type="application/xhtml+xml"/>
...
and ver. 0.4 has:
Code:
> <meta content="cover.jpg" name="cover" />
> <meta content="0.4.0" name="Sigil version" />
...
> <item href="toc.ncx" id="ncx" media-type="application/x-dtbncx+xml" />
> <item href="Styles/style001.css" id="style001.css" media-type="text/css" />
> <item href="Text/content0001.xhtml" id="content0001.xhtml" media-type="application/xhtml+xml" />
...
and so on. I don't know if the order matters to some reader software but not others.
If you're comfortable unpacking and repacking epubs, you might try substituting the 0.3.4 version of content.opf into the 0.4 epub, and see if it then displays the title. I don't have a nook, so I can't try it myself.
I did try them on ADE, firefox add-in, and Calibre's reader, and everything looked the same (i.e. title present) on all readers. So it might be a nook thing.