For DVD playback, definitely. ATI cards have arguably the best DVD playback out of all video card manufacturers without a dedicated DVD solution. For the Mac RADEONs, you'll get the ability to use Adaptive De-interlacing to improve the image quality when you watch non-progressive scan DVDs (a very large percentage of the Anime out there).
Much of the DVD video decoding is offloaded to the Video card. More so than when using NVIDIA chipsets. So, DVD playback will utilize less CPU.
As for games, it depends. The NVIDIA drivers are highly tuned for Quake III and other Q3 engine games. IMO, ATI has better overall compatibility.
ATI has more features enabled in their drivers, such as FSAA (I've heard rumbles that NVIDIA cards will be able to do this soon, nothing concrete though) and TruForm (only with the 8500). And it has features other cutting edge hardware features that only the GeForce 4 Ti will have enabled among NVIDIA's cards (on the Mac. For some reason, some of the GF3s features will not be supported).
What type of games, or even what specific games do you play and intend to play. That can make a difference in which card you want.
But check out other sites for benchmarks (unfortunately, they're all pretty much Q3 scores).
GF4MX vs. GF3
GF4MX vs. GF3 vs. GF4Ti
GF4MX vs. GF3 vs. RADEON 8500 vs. Radeon vs. Radeon 7500 vs. GF2MX
I personally stick with ATI cards because they're more active within the Mac community, more frequent driver updates which also support older cards, and better DVD playback, and, at least with the 8500, price/performance is just fine.