Having owned just about every color of leather, I find that black doesn't get any hotter than any other color. It does get hotter a little quicker, but it's a very small difference. Window tint makes a much bigger difference than interior color.
As for leather vs leatherette, the X1 leather quality is pretty low, it's highly waxed with closed pores, so no amount of leather treatment will change it. It does breathe better than leatherette though and I find that you can feel the seat heaters a little quicker too. It's a small difference, choose whichever you feel like, it's not going to effect your enjoyment of the car.