If Apple comes out with a 14 inch version, and after a year or two of working out the new software-ARM kinks, it just might be a compelling buy - especially if you're into the iPhone-Mac system combo.
As soon as you buy one, then Microsoft will release its own ARM chips and put them into their high end systems, but I don't see Microsoft really competing with Apple in gaining any market share. In all likelihood, if you buy an MS system, within 5 years Microsoft will stop manufacturing them for lack of sales - just like with the Windows Phone. Once that happens, the Microsoft ARM systems will turn to crap as they'll be abandoned by the software developers - just like what happened with Windows Phone.