This Speedmaster Pro has a white gold bezel ring:
The difference is barely visible, only in direct sunlight, I have the impression of the white gold being a tiny bit more yellowish.
The bezel has collected several nicks, but I assume this is more due to the unique shape that exposes it to all kinds of bumps, than to the relative softness of the material.
Generally, I think the white gold vs. steel issue is tradition. Fine and complicated watches normally were not available with steel cases, but in times of economic recession, many people preferred (especially in Central Europe) not to show their wealth too prominently, and thus white gold (later platinum) gained popularity.
Regards,
Marcus