That's a big question! That all depends on what you're comfortable working with and what you're going to be using the computer for.
The consensus these days seems to be that for 3D, you're better off going PC because you can build a FAR more powerful system and it will be cheaper than a Mac.
But, if you're not doing "hard-core" 3D, a fully-loaded iMac or MacPro will be fine. If you're having trouble deciding, we have an article series you may find helpful
Here are some excellent resources to check out for specs:
And for the DIY Build a PC type here's a couple more resources: