In my experience, mountain biking tends to be more challenging than road cycling. The terrain in mountain biking is more unpredictable and requires a higher level of fitness and skill to navigate. Also, the constant changes in elevation, turns, and obstacles you encounter on a mountain bike trail make it a more intense workout. On the other hand, road cycling is more about endurance and speed on mostly flat surfaces. So, while both have their own unique challenges, I'd say mountain biking typically takes the cake in terms of difficulty.