1. How much are you willing to spend?
2. Are you a hobbyist, or a working professional?
3. Do you just work in 3d, or do you do work with other software/hardware requirements?
4. Are you competent enough or do you have a competent friend or someone you can pay to maintain/support/upgrade/troubleshoot your hardware/software configs?

That last question is the most important with any new hardware/OS choice because simply buying any prefabbed box that is not specifically designed for what you want to do now, and in the future will only cause you expensive headaches and ultimately wrongfully placed anger or even hatred for a given manufacture of hardware/software or OS.

Also keep in mind that by limiting yourself to a given OS can have dire consequences in the event you want to upgrade or play with drivers as all OS's have varying levels of driver support and issues.

Also, as this is a technical question, I moved this thread to the correct place