Choosing the windows or linux platform for installaing SBR is based on customer requirement since this also involves communication with other backend servers liker LDAP, AD, SQL , RSA etc.
If this is for SBR enterprise edition and if you want to use SBR for authenticating using a windows based domain controller then selecting a windows 2003 /2008 server for SBR installation would be fine.
Once you finalaize your deployment scenario with the backend servers infornmation you can work with you local Juniper SE in choosing the right platform for instllaing SBR on linux/windows.
Hope this resolves your query
If the environment has an active microsoft domain, you can choose SBR enterprise edition 6.1.7 on a winsowa 2003 or 2008 server.
Hope this clarifies your query.