Help me buy a notebook

I'm back again helping someone else shop for a new computer. The person in mind is a student with a proposed budget of $500-700. Currently we are looking at the 11" MacBook Air because it's thin & light, but that's only because it's been difficult to find many good ultra books at our local retailers. I did consider the IdeaPad Yoga, but the reviews pointed out a lousy keyboard and battery life.

The laptop should be comparably thin and light to the MacBook Air, have a modern Intel processor, a decent battery life (8+ hours) and be available in 11" or 13". I would prefer a metal frame, but a good plastic model wouldn't be turned down.

Thanks in advance for any help, it can be daunting to investigate every single model out there!