Chromoly is high carbon steel, regular rifle barrel stuff. Chrome lining is a plating process that coats the inside (bore) of a barrel to help prevent damage from corrosive ammo.
The hard chrome lining is not distributed as evenly as you might think and does have variations in plating thickness that will affect bullet friction and accuracy. Still, great stuff to have in your barrel.
Stainless steel on the otherhand does not require a chrome lining as it is already corrosion resistant. It provides a more consistant bore dimension than chrome lining a barrel. And it's heavy.
If I had the option of SS over chrome lined in the same barrel contour and dimensions, SS all the way!