As a combat sports fan who follows the issues with brain trauma (chronic traumatic encephalopathy, or CTE, being the condition to watch out for): it's gonna be both.

We used to think it was the major concussions that did the long-term damage, and you'd be fine if you avoided the big shots as a boxer (let's say). Modern research shows that the sustained, repeated, sub-concussive blows are genuinely bad for the brain. Training itself can be a major cause of brain damage.

Soccer doesn't focus exclusively on heading the ball, but practicing heading for years, and pushing your limits in games, definitely adds up and has resulted in soccer players with CTE. Major concussive events, like head-to-head collisions, are decidedly no help on top of those training regimes.