Research suggests that “sustained aerobic exercise might be most beneficial for brain health also in humans.” ....These results do not mean, however, that only running and similar moderate endurance workouts strengthen the brain, Dr. Nokia said. Those activities do seem to prompt the most neurogenesis in the hippocampus. But weight training and high-intensity intervals probably lead to different types of changes elsewhere in the brain. They might, for instance, encourage the creation of additional blood vessels or new connections between brain cells or between different parts of the brain.
So if you currently weight train or exclusively work out with intense intervals, continue. But perhaps also thread in an occasional run or bike ride for the sake of your hippocampal health.