I'm wondering if there will be a materials or texturing difference between the two as there was on the Nexus 5. If not, my general preference would be Midnight Blue, but if the 64GB in that tone is sold out when I'm ordering and the Cloud White is available, I wouldn't hesitate - especially since it'll be going in a shell case anyway.
Kinda disappointed there's no black option (although, I'd bet by early next year, there will be another color or two, one of which will be black), but I'll go with the blue instead. If worse comes to worse, and white is my only option, I'll live with that, but I prefer darker colors.
I slightly prefer white over midnight blue but I would have strongly preferred either an all black or an all white phone. The all white moto X looked so nice this year too (or at least better than 2013). At $650 they could have at least had matching colors for the front...