Just a quick note that as of the start of this month, Microsoft is no longer selling Windows 7 and Windows 8.1 licenses to OEM PC makers. This means that once stock runs out, you'll no longer be able to buy a new PC with one of these two operating systems.
While there's still a big group of users that clings to Windows 7, for the vast majority of consumers Windows 10 is definitely the better choice.
As you can see, as of today, the only Microsoft-developed OS that you can get pre-installed on a new PC is Windows 10. Luckily, Windows Vista, 7, and 8.1 are all still supported, and will continue to be for some time.
The end of extended support for Vista will be in April of next year, and for Windows 7 it will be January, 2020. Windows 8.1 will still be offered mainstream support until 2018, and then extended support won't end until 2023.