Which STS you choose should really be based on your near term as well as strategic requirements. For the record, PingFederate works very well in a Windows environment. Ping Identity actually has many, many Windows IT shops successfully deploying PingFederate. So whether or not you are a Windows shop shouldn't be the sole determiner.
You might want to consider the range of applications that need to integrate into your SSO ecosystem. That includes both identity management products such as Active Directory, OAM, SiteMinder, as well as relying applications. Your SSO solution should be selected based on those integration needs.
Ping Identity did a ton of work this summer to ensure that we are well aligned with ADFSv2/WCF/WIF environments. Along with that, PingFederate provides very rich integration into many existing identity management products, directories, and databases. There are also several language kits for embedding SAML, WS-Trust, and WS-Federation single sign-on into .Net, Java, and PHP.
Another key differentiator between PingFederate and other SSO/Federation products is the speed and ease with which a PingFederate ecosystem can be established. Generally, PingFederate customers get up in running in hours or day. Not weeks or months.
Also note that Ping Identity offers a cloud based solution, PingConnect, for those companies that are not ready or don't need to invest in a full scale on premise SSO/Federation solution.