This doesn't mean you can't buy a MacBook Air if you want one. The huge gap has closed dramatically with Excel for Mac 2011, but it doesn't have exactly the same functionality, plug-ins may not work, and you'll find it harder to get help. Although Microsoft Office is available for both Windows and Mac OS X, I would recommend against choosing the Mac version. While I have no idea which programs might be needed, it's a good bet that your course will include Microsoft Excel spreadsheets, including programs written for Excel.
You may need to run specific business software for some parts of your course, and if your laptop can't run it, then you will probably need to run it in the university's computer labs. In this case, the best source of advice would be economics department staff at the university you're going to attend. If you are buying a laptop for a specific purpose, then you need to make sure that it does the job. However, the MacBook Air seems to be a better contender. I have recently come to like very much the Asus Zenbook Touch UX31A. I would like a decent, light, fast and portable laptop. I'm off to university – my first year for a BSc in Economics – and I need advice on which laptop to get.