The difference is that SiteAssist pro does all of these things at once in one step.
In your current workflow CSS Sculptor can be used to create a page, then you can use it to create a template, then you can add menu writer, then you can create your pages and then update menu writer to match your site content.
SiteAssist pro does all of that for you in one wizard. It even starts populating the contents area of the css sculptor derived template with pre-designed layouts that you can design and add to yourself.
So it really helps when creating an entire site all at once and maintaining the pages and links to those pages so that they stay in synch, and allows you to keep standard contents outside of the layouts and menus.
In terms of the end result, there is little if any difference.