Whey protein is ideal when you want a cleaner macro profile with higher protein and fewer calories. It is usually the first choice for recovery and lean muscle support.
Mass gainers are useful for hard gainers or athletes who struggle to eat enough calories through food alone. They combine carbs, protein, and sometimes extra fats for a more calorie-dense shake.
Choose based on your eating pattern, body goals, and recovery needs instead of trends. The best supplement is the one that actually supports your full nutrition plan.