Depends on how much power you need. If battery life is the priority then macbook air wins every time although I would wait until broadwell is out in laptops as this will make it possible for laptops to have fanless design. But if you need it now and want a windows alternative then the asus xl55ln I think its called or xl55la, one has a discrete graphics card and the other doesn't. Lenovo y50 is another alternative.
I would second Joe's opinion, the mac being more expensive, Lenovo and Asus have some affordable options and high end, not sure what your budget is our what u are using it for
Depends on how much power you need. If battery life is the priority then macbook air wins every time although I would wait until broadwell is out in laptops as this will make it possible for laptops to have fanless design. But if you need it now and want a windows alternative then the asus xl55ln I think its called or xl55la, one has a discrete graphics card and the other doesn't. Lenovo y50 is another alternative.