Do I need a different helmet for road vs. mountain biking?

Not strictly, but reviewers note that MTB-styled helmets (like the Giro Fixture II) typically add a visor for sun and branch deflection plus more rear-head coverage, while road helmets prioritize ventilation and aero. For mixed use, a trail/all-road helmet with a removable visor — a configuration several picks here offer — is the most-recommended compromise across the cycling forums we read.