Windows 2003 and Linux/Unix hosting are almost at the same stability and generally depends on how they are managed actually.
If you don't need asp or asp.Net, linux hosting would be much cheaper for you because of licence fees of Microsoft.
To make sure which costs less, you can check out web hosting directories such as
www.HostSearch.com,
www.***************, or
www.HostPulse.com
Or you may do a direct Google search as well