With OpenID, every user of a particular app has to have his or her OpenID registered for that app. That can get tedious if you have to turn on OpenID for hundreds or thousands of users. With SAML, it is much easier, since it works with X.509 certificates and you can enable an entire user population at once.
Most of the SSO tools support SAML and make it relatively painless to import a batch of user IDs and have them setup on the system. You bring up the web-based admin portal and run through the process to upload your group of users and set them up for specific policy groups to share a common set of apps.