Portal (with the same code base across platforms) is one fifth as likely to crash on a Mac than on Windows.
Windows will have nice clear images while macs will have poor grainy images.
No, I have a Macbook pro and a mac pro (mac with best processor + graphics card). Mac pro could probably play it better than pc. Heck, an Imac could play Portal better than a PC.
Mac can play portal better.Windows can play everything else well.
Here is a link to the article that I read and found one of the facts inside to be quite interesting http://store.steampowered.com/news/3837/ Now stop raging at macs. They can run Portal better than Windows.