Hi Andrea Sitefinity looks to be a fairly powerful CMS solution based on the .net framework. Sitefinity is made by Telerik which means it's not free nor open-source. I haven't used Sitefinity however I have used Telerik controls in both DotNetNuke CMS deployments and Windows forms applications.
I've used many a CMS and prefer DotNetNuke for a .net based CMS. It has all the bells and whistles as any other CMS and enables the SEO work you need to do. The SEO team I source my projects to have given great feedback about the ease of adding and integrating the technics and mechanics of SEO components within the DNN framework. The results, albeit more to do with linguistics and less with on page mechancis, have yeilded impressive first page positions in the major search engines for very competitive, target key phrases. In many of my clients' cases, this solidification of SERPs positioning has lasted 4 and 5 years.
So while I can only comment on the value of Telerik's interface controls, the best .net based CMS, inclusive from an SEO perspective, is DotNetNuke. It's also free (open source), supported by hundreds of thousands of community contributers and members, and has added a paid support option if you need accountability and immediate answers to questions.
I have deployed DotNetNuke for small business, large corporations, government, education and non-profits in both Canada and the US.
Hope this helps..
Mike.
