Both Linux and Mac OS is good for web development. I think it depends on hardware and tool chains you are using. Usually, Linux and Mac OS have the same tool chains, like Eclipse+Rail+Gulp+Bower+NodeJS. So OS is not a problem, hardware is what you should be concerned about. It will save lots of time if you have a high performance computer. and I think the hardware of Mac Book is better that most of other PC/laptops.
I have a Thinkpad E420 laptop which has Windows 7 preinstalled. I upgraded its memory to 4GB. But it is still not enough in windows 7. so I installed Linux Mint Xfce. It is really fast and takes up little memory. But the CPU and disk is not good enough, I still have to wait a time when opening Eclipse or Firefox. So now, I'm planning to buy a Mac Book.