ahhh exactly my decision scenario a few months back...
the x201s is a fantastic machine, period. it's supremely quiet, efficient, light --> well engineered. it's definitely the more durable machine with its matte rubberized finish and the magnesium/carbon frame.
i chose the 13" mba because they no longer make the x201s. the x201s has the higher resolution screen. not sure why they dumped the better screen.
it'll really going to come down to a lifestyle decision than hardware/os. i'm not a fanboy of either microsoft nor apple, but i gotta say, all the griping about the windows OS within the apple camp is unwarranted. windows 7 really is a wonderful OS so kudos to ms for that. OSX has strengths in the areas of hardware/software integration like the trackpad and sleep mode. in terms of manipulating windows, organizing files, and simply getting work done, i believe win7 has the upperhand.
so if you're a college student, DJ, a graphic artist, apple may be better for you. if you're a traveling professional that uses enterprise applications and lead a lifestyle where surfing for the latest itunes tracks or messing around with panel color profiles is less interesting than making your six figure bonus, then lean towards the lenovo.
apple --> leans towards the consumer mkt
microsoft --> enterprise mkt dominant
honestly, if the x201s was outfitted with the latest trimmings, i'd buy it today and sell my mba. with that said, the new mba should be a proud moment for apple. along the hardware integration with software front, it kills just about any other piece of mobile computing gear out there.