This is a really hard choice, I used XP for years and then Windows 7 for years. I started Windows 7 about a month or two after it was released, and still on xp then. Only time I have used XP is setting up an shitbox to play around. Moved to Windows 7 since xp was not supported.
Windows XP is still one of the better softwares that was ever made. If it was still updated, I might use it over this crappy Windows 10 thing. Windows 7 is probably the best, since it's actually being updated. Although, I do still have an XP desktop that I use to play some old XP games.
Windows 7 as it's newer (more update to date), still is being supported officially, isn't built on a framework that is more than 10 years old, and it provides features and functionality that they only dreamed in having in XP.
Dual-boot is the way to go! I'm currently dual-booting Windows 10 and Windows XP. Even if I were still on Windows 7 for my main OS, I would have Windows XP as well.
Windows XP will probably always be my favorite OS. With that being said, times do move on. So now I'm using Windows 7. I haven't upgraded to 8, 8.1 or 10 and I have no plans to do so either. Windows 7 is awesome and my machine is a beast.