One study indicated that male long distance runners experience a decrease in testosterone levels. Associated with hyper aggressiveness, this hormonal change may very useful to some. On the other hand, the opposite seems to occur with high intensity training. Spend time in any busy weight room and you can almost feel the testosterone being pumped up. The high one gets from lifting weights is not the same as the tranquility experienced by those who condition aerobically.
Weight lifters feel powerful and confident following workouts.
Weight training appears to be be more effective in reducing the risk of osteoporosis. From a mental health perspective I would recommend cross training in both aerobic and anaerobic exercise to increase variety in workouts while encouraging feelings of both powerfulness and tranquility. The frequency and duration of exercise is determined by ones goals. To get in shape quicker it is recommended that one exercise frequently as opposed to fewer times and longer durations. Weekend athletes don't cut it.