The hub spacing (120mm) doesn't have anything to do with the chainline since the spacing measures the distance from locknut to locknut and is adjustable, while the chainline of the hub measures from the center of the hub to where the cog will sit. If you are using a track hub with spacers, get the 103mm. If you are using a road hub or mountain bike single speed hub, find out what the chainline of that hub is and adjust the bb accordingly.