How much does a climbing bike actually help compared to an aero bike on a hilly course?

On a course with 1,000m of climbing, a climbing bike that is 1 kg lighter than an aero bike will save roughly 30–60 seconds on the climbs. However, the aero bike will gain back time on descents and flat sections. The break-even point is typically around 3,000–4,000m of climbing per 100 km — above that, the climbing bike is faster overall. For most hilly gran fondos with 2,000–3,000m of climbing, the time difference is small and personal preference should guide your choice.