I have CroMo and Stainless Zeros on a couple different bikes.

The CroMo ones are painted black, the stainless ones are silver. Over time, the paint will chip off the CroMo and the spindles will rust a little. It's not a lot though.

I do find that I prefer the appearance of having the spindle match the crank arm, i.e. black spindles on black crank arms and silver spindles on silver crank arms. But if cost is an issue, just get the CroMo.