These are some reasons to use a MacBook (Pro):
- Build quality is very good. There are only a few PC brands with high end models that can compare with the Mac. And yes, they are similarly priced.
- OS X is very good. While Windows has its quirks and driver issues, 32 versus 64 bit systems etcetera, OS X just works.
- No hassle with anti virus software.
- UNIX based, which makes developing for UNIX based systems like Linux much easier.
- Must have for developing iOS apps.
- Must have for debugging Safari on OS X.
- Must have for debugging Safari on iOS.
- You can run OS X and Windows in a virtual machine at the same time.
- When typing commands, bash is much better than Windows CMD.
- Free or very cheap OS upgrades.
- Packaging software like Homebrew or MacPorts makes installing open source software very easy.
- App Store with lots of well designed and rated software.
If you only need the have one tool to make a living, don't you want that tool to be very good?