Defensive wall is good at blocking shots from free kicks
Shadow striker has good shooting stats and will score you a lot of goals.

But if there is for example a 5* player with no special ability and a 4* player with defensive wall special ability, I'd go for the 5* player. Of course it's better to have a 5* player with defensive wall but it's all down to their specific stats

My best player specialises in corners but is excellent at taking free kicks, so it's all down to their stats.