Some people are over concerned with how beautiful the HTML is as opposed to how it appears on the reader. Calibre makes good looking epubs out of practically anything. But its HTML is not beautiful. Does the non-technical reader care? Not at all.
Since you have a Sony, you will want to check it on your PRS-350 to make sure things that look good on the computer screen look good in the ebook. You might download the older version of the reader for PC software from the Sony Reader site. It works better in my unhumble opinion for checking than the new software does.
One caution if you go back and forth between them- Sigil does not grab ownership of a file once open, so you can open it in the Sony Reader for PC at the same time. The Reader software does grab ownership and will cause an error in Sigil if you try to save while it is displayed in the reader. Just clicking on the list, rather than display of a particular epub will avoid this conflict.
Last edited by mrmikel; 01-13-2013 at 12:40 PM.