Unity up to 5.2.5 works on/for XP. For the UE4 you need 7 to make it work and while it can supposedly build for XP, I couldn’t get this feature to work when I tried. Now that is probably my main reason and probably not relevant since I am the last person standing against microsoft’s despotism. But there are others reasons that may talk to the other folks.
Unity comes with an IDE that is quite performant. The UE4 defaults to visual studio, that comes separately and is an humongous piece of **** (and I’m weighting my words).
And probably what’s the most important, Unity’s compiler is fast enough that it allows to make your game through trial an error. On the UE4 a slight change to the code base and it takes forever until compilation is done, which makes the engine only usable to people of experience who know where they are going and how.