For max you will most likely be better off with the dual pIII's. I build computers for people that all they want is 3dstudio max rendering powerhouses. They go with duals and a lot of ram. I would suggest to you to go dual athlon. I like both intel and AMD, and when it comes time to choose I always way the pros and cons of both. And for you, wanting to run max, AMD's running max are unstoppable. It is by far suppierior and you can't get better than dual AMD for 3dstudio max. When compared to dual xeons many bench marks the xeons take and many the amd's take, but always in max the AMD comes on top.
If max is your main reason for the comp dual AMD is your only option and the price isn't very outrageous, considering the level of computer you're building. One more thing to note when building, AMD only supports their MP chips in SMP mode, but all the dual systems I've built I used XP chips and they were recognized as MP's. There aren't really any differences just that AMD supports them (they cost a bit more). Sorry I'm giving you a third option when you only asked for the one out of two, but I know I've saved others hours if not days of rendering time by building them dual AMD systems.
Good luck