Windows 10 from 7 or 8.1

Hi,

I am going to upgrade to Windows 10 on a new SSD. I am currently using Windows 8.1, but there are issues with my PC currently, added to which the SSD my OS is currently residing on is short on space.

Before I do this, I know I have to load either W7 or W8, and update with the latest updates, until I get the notification in the Task Bar.

So - as I have Discs for both Windows 8 & Windows 7, which would be the better one to use as the base OS.

I'm thinking along the lines of using Windows 7, as I no longer use this OS on my PC, so the serial number is "free".

Would it make any difference? which I use? (I daresay using the Windows 7 way will involve me in loads of more updates)

What would others do?

Thanks

Windows 10 from 7 or 8.1