Active Directory

I have 2 DC’s in the parent domain (root forest) and the 1st installed obviously holds all the FSMO roles since it was the 1st dc installed in the 1 domain. So with that in mind and I added the second domain controller, is it best for this server to be the infrastructure master or another global catalog server. I need this server for fault tolerance in case the 1st goes down but have read conflicting white papers on this. We don’t have the option(budget) to install a third dc as the catalog server…..