Welcome to the forums. Install Windows 10 OVER Windows 7/8 once, that will permanently register your machine with Windows 10 for free.
Then you can either go back to Windows 7/8 or have the option to go up to Windows 10 at any point in the future if you wish.
Newer Windows 10 builds may allow you to install Windows 10 using a Windows 7 key.
In either case, it's a smart move to install Windows 10 before July 2016, so that you have a free *choice* between the two.
I wouldn't go out of my way to spend money on Win7 though, unless you have a specific reason.
Windows 8 + Classic Shell is identical to Windows 7 in every way and is snappier and I would choose a Windows 8 + Classic Shell combo as an option to Windows 10.
Things that are problematic in Windows 10 are not problematic in Windows 8... but they may be insignificant to a lot of people.
Windows 8 has an option for Windows Media Center, has less privacy issues, does not force install each and every update (including optional & unnecessary updates) the moment you try to check for Windows Updates (like Windows 10 does)... things of that nature.