I agree with William and Malachi as well.
WordPress is the world's number one CMS. It is free, offers the most extensions and support, is easy to use and you are familiar with the software. If you ever need work done on your site, there are plenty of experienced developers who can help.
I have reviewed about a dozen CMS solutions and have never heard of Sitefinity. It may be a great solution but even if it was, your selection of themes, extensions, updates, etc. will never be close to what WP offers. If your current developer left you for whatever reason, your list of experienced developers who can work with you would be severely limited.
My solution would be stick with WP unless you are provided with an exceptionally compelling reason to move. If that is offered, also be sure to understand what WP offers that Sitefinity does not. The main reason I would use another CMS rather then WP is because WP is best for blogs and simple sites. If you require more features, my preference is for Joomla.