It was explained countless times but here is my take on it:

Zonal marking is solid defense. It is good if your team quality is similar or worse than your opponents, or your opponent is playing very aggressively. Aggressive mentality usually don't work well simply because most players choose this mentality as default. It also require your team to spread out evenly on the field, as unmarked position in any area of the field, even on the opponent's defensive line, makes it extremely vulnerable to counterattacks.

Man to man marking offer better defense against counterattacks, thus it work best if your opponent plays defensively. As many experienced managers play defensive counter against stronger opponents, it is usually effective if your team is of better quality. It is also more flexible, that formations with holes in its defense it may be used successfully.