Apple has traditionally switched back and forth all the time, depending on whoever was offering the best option at the time--earlier PBs used NVidia chips, current MBPs use ATI, early G5s offered ATI as the top of the line, current ones offer NVidia. This makes the most sense, as they basically go back and forth.
Sticking to only one or the other would be a not-so-smart move, just because it limits options.
That said, I'm generally an ATI fan, for two simple reasons:
1) ATI has been kinder to the Mac in terms of aftermarket drivers--their ATI control panel has no NVidia equivalent, and you also can't software overclock an NVidia card (ATICellerator etc) on the Mac so far as I know.
2) ATI cards have recently been less prone to being dual-slot monsters that chew up an ajacent slot. But that could, of course, change at any time.
Really, I'd say go with whoever is best, but it just seems that ATI has, on average, put a little more effort into the software end of things for their Mac products recently. There's also that dual-platform Radeon card, which was kind--no equivalent product from NVidia at all.