I have a Mac and a PC and I feel more productive on a PC. I think the main reason for this is that I'm a "maximised windows" kinda guy and switching between windows and tasks feels much more smooth and simple for me in Windows. It may sound like a small thing but because the OS X dock deals with apps, not windows, it can be a pain switching between open windows (yes I'm aware of shortcut keys etc). Also the tendency of OS X to clutter the screen with open windows (which it seems most people are happy with) really frustrates me. Spaces alleviates this somewhat, but I much prefer something like Photoshop on Windows, for example. Much cleaner.
Having said that, the shell in OS X is really useful and a lot of Linux stuff you'll be using (Python for example) feels nicer in that shell than under Windows. Others here have commented on the "unix core" and it definitely is a benefit.
For me, though, I'm content to use Windows and Putty/VNC into my Linux box when required.
If you can, play with a Mac before you buy it. Developers tend to be a picky bunch so what works for others may not work for you.