Speedy, Eriksen is not as much of a project as Zaha. He has already had two seasons as his team's most important player in the Dutch top flight and has had some boss performances in the Champions League. He'd come to Arsenal and be a starter right away.
When I look at Zaha, I see someone who is at Oxlade-Chamberlain level at best. Look at Ox's performances and tell me you want someone else at that development stage in the frontline right now. We should get really good players around Ox to help improve him rather than bringing in more projects