Macs are strong in 2D and will get by in 3D with packages such as Maya, Lightwave and Electric Image. The x86 platform will give you a lot more options (more program choices) and higher performance rendering at the high-end, especially will multi-processesors.
If you're starting from scratch, definitely go x86. If you already have some high-end G4s, then it depends on what your goals are. You can invest in some Mac 2d and 3d packages, and get the job done. If you want the maximum speed and output per dollar of hardware/software, you still should go x86.
Another issue is whether or not the people working on the machines have Mac-only experience or PC experience or both. If they're Mac only, they would initially be more comfortable and productive on Macs, obviously. But these days, many of the application interfaces are very similar across the 2 platforms, and the only real learning curve should be getting to know the nuances and quirks of the different OS, and maybe different keyboard shortcuts.
Until, recently I worked in a creative production environment full of Mac lovers for a number of years. I never had anything personally against Mac, and I can still work pretty well on Macs, but the last one I owned was in the early 90s. In my office, I always argued for new purchases to move towards PC based systems. Sometimes I won out and sometimes they went with their old favorite, Macs. They never won me over or convinced me that Macs were any better or faster, but I have to say that they did get the job done. The work was weighted towards 2d, but we also did some 3d.
Some people always have an "eeeew" factor when they try to use PCs coming from a Mac-only background. For someone who has worked 12 - 14 hours a day on Mac for years, they get very used to the environment and have all the shortcuts and workarounds down, like a part of their subconscious. Maybe you could liken an artists transition from Mac to PC to a jet fighter pilot trying to move from flying MiGs to F-16s. Certain creative cliques, feel that PC-users are somehow less creative. Of course, if you look at the installed base of PCs in the hands of creative artisits, you'll see that the opposite is true. And really, with so many PC users, you get tons of plug-ins and other user-made add-ons to solve problems and add functionality on the PC.
In the old days, Macs were dominant in the Desktop Publishing and 2D artistic fields. At some point in the 90's, PCs reached parity with Macs, when practically all of the dominant Mac artistic applications reached a comparable level of performance and stability on PCs. Now, PCs clearly surpass Macs in performance, the cost of hardware components is lower, there are more diverse options available and far more commercial software options, not to mention shareware and open source utilites.
For me, there always seems to be one more tool available for x86 machines, that doesn't run on Mac.