Mac Book only if you want to waste money or are totally in love with Mac's. At this time neither Axad nor Revit works in native Mac mode and both require loading Windows through parallels or similar or running Boot Camp. The first option slows you down and the second means you have paid too much for a Windows machine since the Mac functions are not accessible without a reboot.
If you're concerned about rendering speed, an i7-2920XM is way better than the previous generation's i7-940XM, and will give performance almost up there with an i7-980X desktop. If you don't care much about rendering, then target a fast CPU with fewer cores, such as the i5-2540M.
Also, keep in mind that Revit wants lots of RAM.
Search is always a good option:
How do you want to use this laptop? If you want ANY production at all, you're looking at a min 17" screen.
Revit is not available for Mac but AutoCAD is, albeit as a first-generation product with all the limitations and problems associated with it. But otherwise seconded on the Mac opinion - if its important for others to see a fruit logo on your computer, get one; otherwise get a PC.
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