OSX is ... finicky ... with dev tools. Apple does the compilers, and the bolt-on distributions (brew, Macports, fink) seem to have trouble keeping things compatible. Linux distributions don't end up falling behing their own compilers every time the compiler gets updated. I'd rather stay off the bleeding edge compilers, and have everything just work.
There is the "Apple tax", but it's hard to find a good lightweight laptop which is much cheaper than a MBA. A few hundred dollars cheaper, but you lose that on the resale value. For a desktop, there's no comparison (a $600 white-box will thrash an iMac, and if you pay for a nice monitor it's better to keep it than sell it bundled with an obsolete machine).