Connecting two remote sites

HI

I want to connect two remote sites (one is HQ and the other is a branch office) so the remote users can login to the HQ site. On each site there is a Windows Server 2003 Standard Edition installed.I want to conect these sites with routers using ISDN, Dial-on demand.I want this line to be used only when there is a traffic between the sites. My question is: Is it better to make one site child domain of the other (if is it so, it holds my ISDN line always open) OR to use the server on the branch office site as a Remote Access Server, OR some other solution