I personally found the Macbook Pro screen to be irritatingly glossy... I couldn't use it in bright areas. It's a great machine but I didn't find it very practical to use in an environment with bright fluorescent lighting such as a typical office - It's what made me switch back to Windows after being a Mac user for 9 years (I wanted a laptop with a bit more oomph than the Macbook Air offered).
I bought a Lenovo T420 (the smaller version of the T520) and I couldn't be happier. Cost me significantly less than an equivalent mac, is built solid (likely better than my old mac), Windows 7 still isn't Mac OS X but I haven't had any major issues. I'd imagine that the T520 would be fairly large to move around though..
Then again, if portability is an issue, either the Macbook Air or the Lenovo X1 would be your best options. If the laptop will be going between home and work or to job sites often, go with one of those two.
Depending on the nature of the work, having used both Windows and Mac in the past year, I suppose my decision would come down to this - anything more creative such as graphics work or video work, i'd probably go with the Mac, and for standard office productivity applications like Word, Excel, etc, the PC wins out (but just barely - Mac really has caught up in this area and is near equal now).