John should consider mediation because it is conducive to maintaining and potentially preserving the ongoing business relationship with Michael, who is a frequent and valuable customer. Mediation's collaborative nature helps both parties reach a mutually agreeable solution without the adversarial aspect of arbitration or litigation. While Options A and B provide benefits of mediation such as speed and confidentiality, they do not address the crucial commercial reality of preserving business relationships in this scenario. Arbitration, while confidential and allowing expert involvement, is more adversarial and could damage the long-term relationship between John and Michael, making Options D and E less suitable. 

Key Point: In disputes involving ongoing business relationships, mediation is often preferred due to its ability to foster collaborative solutions and preserve professional relationships, an important consideration in commercial contexts within the law in England and Wales.