Windows 7 upgrade could take 20+ hours (reasons to do a clean install)
Now, reading those two descriptions, you would think that the latter would be faster and easier. And indeed, for many users, it is easier. But it's rarely faster.
In fact, in recent tests, it turned out that it could take more than 20 hours to upgrade a system from Windows Vista SP1 to Windows 7. To be fair, it could also take as little as 27 minutes. But if you've got a ton of data and applications loaded on your computer, you might be better off backing up your data and/or partitioning your hard drive and doing a clean install.
The test systems that had the fastest upgrade times had 20 applications installed and about 70GB of data stored on the hard drive. On these systems, it doesn't take much longer to upgrade from Windows Vista than it does to do a clean install, even on a relatively slow computer.
But if you want to check the test results and see if you can spend 20 hours upgrading your system all you'll need is a computer with 650Gb of data, 40 applications installed, a 2.6GHz AMD Athlon 64 X2 Dual core 5200+ CPU, 2Gb of RAM, a 7200RPM hard drive, and the 32-bit version of Windows Vista SP1.