Although of course if I start on one I can always get the other later and form a badass alliance of Men and Elves because, why not? The rules as I have seen are designed for me to be able to take whatever I want. Who can say that's a bad thing?
Indeed. I forsee my Stormcast Eternals having a unit of Ironbreakers and Slayers along with then, and an Elf Prince on a Dragon or Griffon and perhaps a Mage.