If you plan on using Windows, I'd get the XPS 13. MacBooks get significantly worse battery life running Windows than Mac OS and you're better off with a notebook designed for Windows. If you prefer Mac OS, then wait for the new MacBook (it looks like there will no longer be an "air", it will just be MacBook and MacBook Pro).
If you're worried about build quality with the XPS, don't be. I have an XPS 15 and an 11" MacBook Air. The XPS is just as well built as the Mac, if anything it's a bit more sturdy because the aluminum is thicker.