RE: DELL ALIENWARE AURORA R1, R3, X51, and XPS 8700
First there is a free upgrade from Windows 7 OEM to Windows 10 Retail so there is absolutely zero point in purchasing a Windows 8.1 Retail license...
Windows 10 TH2 is essentially a fixed version of Windows 8.1 where Metro Apps work correctly as "Windows".
Almost any system shipped with Windows 7 can run Windows 10. As they are 2009 and 2011 models respectively Dell did not provide updated drivers for Windows 8.x or Windows 10 because they are all out of warranty. All the drivers should be inbuilt into Windows 10 TH2 or automatically obtained via Windows Update. Its advised to update your BIOS to the latest revision before proceeding.
A Clean Installation also in general performs better. See here for details:
Both systems likely have a legacy (non-UEFI) BIOS so take that into account when making the Bootable USB. Although you may want to look at the BIOS settings for the 2011 model to see if there is UEFI support.
Dr Philip Yip Tech Enthusiast and Author of the Unofficial Dell Windows Reinstallation Guide