Hi Bhushan,
yes, NZDT is a service that is offered excusively by SAP Consulting.
Which approach is more suitable depends on many factors, e.g. hardware availability or downtime requirements. With NZDT, a clone sever helps you to get rid of the upgrade downtime. With downtime-optimized DMO you save the money for the clone but the entire upgrade downtime remains part of the business downtime. SAP Consulting supports the customer in finding the right approach before starting the actual implementation. You may contact mds@sap.com for more details.
Regards,
Ronald