New colours usually sell pretty well, given the novelty of them. But since there aren't any, from memory it's typically black models with base capacity. So I'd say space grey 64GB would be the quickest seller this go round.
Given the price point of the iPhone X, most will likely opt for 64 GB in either color. But then some have stated in other threads, both the 64 GB and 256 GB will be fairly close in terms of back ordering.