Whether or not to use Forms auth or windows auth is highly dependent on your project, its scope and user base. Your question is very broad, so my answer is also. Also note that they are not absolute and very subjective. Use Forms auth when your usage base is external to the enterprise. Use Windows auth when your company has an active directory and its user base is internal. Use Forms auth if you have a specific set of privileged ones, and infrastructure partners do not want to add 100 user groups. Use Windows auth if there are other internal applications, and you want to implement Single Signon. Use auth forms if you want more control over password policy and user registration. Use Windows auth if your enterprise uses IE and users do not want to request a username / password.
Again, very subjective and simply because you want (for example) Single Sign On does not mean that you cannot use Forms auth. In the end, it just depends.