Actually, it doesn't matter whether you use legs, helmet or shoulders (and possibly, not 100% sure, boots) as second piece since they all offer the same armor...IMO
Looks: Helm/Shoulders
Armor/Resistance: Chest/Legs
However, that being said, I've never noticed too much of a difference in damage mitigation from the amount of armor/resistance you get from the Chest/Legs combo, compared to Helm/Shoulders combo. As such, I tend to go more for that one as most medium Helm/Shoulder styles aren't particularly too attractive (imo).
]Actually, it doesn't matter whether you use legs, helmet or shoulders jand possibly , not 100% sure) as second piece since they all offer the same armor...