The Sony Vaio S 15.5 isn't hard to upgrade, so I'd say go for it if that's the machine you like better.
The Retina MBP isn't upgradable but it ships only with SSDs and solid RAM selections, so there's no worry about having to upgrade to an SSD later. Of course, it is quite expensive, especially if you go for the 512 gig SSD model
-------------------- Lisa Gade Editor in Chief, MobileTechReview