So, the iPhone X is an expensive phone overall and deciding on the color of it could be challenging for sum people. First of all the silver is more like a white color in real life and space gray kind of black. Silver will be better if you will use a clear case, it would look more nice than a black iPhone with clear case. Without case best would be white as well as you will not see so many scratches on the back of it. Black would be a color that it neutral, will not blink to much attention and will look like a regular luxury iPhone. But anyway you could always get a case of what color you want and shape, so because of that the color don't really matter. In terms of if you would like to sell your iPhone in the future i would suggest that silver would be a better sale because of its popularity at the moment. In the end you can't go wrong with either. Let me know what is your preference ?