Consider the weight of each axis, how much force it will take to move it when cutting and how fast the rapids will be. But remember that holding torque (the most common number given to distinguish one stepper from another) isn't the most important thing. You need a motor with low inductance in order to run it at the speed it's capable of, given the power you're supplying. This is usually limited by the drivers you're using. If your motors have 2 mH inductance, you'll need to give it 45 volts to run it at optimum speed. If your drivers can only handle 36 volts, you'll want to get motors with less inductance than that. If you want more torque at the peak speed, get motors rated for higher amperage but the same inductance.