The SONY has much better specs than the MBA and so do most PC box/books over Apples.
But who cares?
The best reason to use a mac is MAC OS X. It runs natively and purposely for MACs and so the system compatibility is always assured. Sure, you can use Snow Leopard on any of these Wintels, but that's more tinkering than most people are willing to undergo themselves.
The fact is that the SONY Z will undoubtedly be obsolete in spec within the Wintel software environment much quicker than a Mac does within the OS X environment.
Basically, that means the hardware of Macs last longer than PCs. That's why Macs hold their value better than PCs, you can use an old macbook for a long time and not worry that it will be rendered completely impractical to use when the next version of OS X comes out.
Imagine the case for the the Sony once the next version of Windows is released... No one really knows if it will still be good enough! I doubt it, remembering the woes of those who had to downgrade from Vista to XP...
*Mac
Somehow, just somehow, I doubt that the Windows 8 will be too advanced for an i7 chip. It probably won't require over 4GB RAM, let alone the 8GB fitted to many Zs.
You certainly won't be choosing an Air for its futureproof-ness. A Pro with the i7, yeah, but definitely not an Air with a C2D.