Limit Group Policy to specific OS

Hi. I'm sure this is a common scenario, but I would like to hear the forum members opinion as to the best way to do this. I have a Domain (currently Server 2003 DC) consisting of Windows 7 and Windows 8.1 workstations. I've created a GPO to control what Windows 8 "modern" apps are available to users, using a PowerShell script. I only want it to run on Windows 8 machines, but I want it to be per user (in other words, I have it set in the user configurations). Do I put all those workstations into a separate OU and link the GPO there, or do I use a wmi filter?

Thanks!