Well this is actually my conundrum.. on paper the specs are not different, but practically it seems they are. I saw three ultrabooks in PC World and only one had an SSD (£900 vs £1100 for MBA).
And the battery life of all of them was claimed as less than that of a MBA.
I REALLY need to replace my laptop, but am in two minds about labouring with it until Windows 8 comes out...
No need to use OS X - there is no software on there that works better with the exception of iTunes (which is possibly the worst well used software ever - any Mac Fanboys complain about Windows, I just need to say iTunes...)