As Hugo said there is no more ATI in the MacBookPro line at the moment, so you probably ask about the X1600 that was there in older MBP vs the 8600M that is there in the actual line.
From what i can remember, the X1600 works fine with 3D programs so it's a good overall graphic card and ATI cards are known to be very good with Apple's Core technology so if you use Motion for instance or programs like this, this graphic card will "feel at home"...
However the 8600M is a more recent card and nVidia's line of product is a lot more powerful when it comes to pure 3D than its ATI counterpart.
So if you are mostly into 3D you should consider nVidia card rather than ATI.
Cheers,
Laurent aka Tartiflette