go for storage vmotion if you are running vSphere Enterprise or Enterprise + ,  if you are not running one of those editions you will have to do it with SanCopy. There is negligible performance impact when you do storage vmotion but benefits of ease of migration and no downtime will surely outweigh the extra disk load during the copy.
We are going to migrate data from CX3 to CX4.  We have previously performed the task when we migrate data from CX300 to CX3-20. SANCopy has been used then.  Currently, we are using vSphere 4.1 ESX hosts.  A consultant suggests that there is risk using SANCopy and he suggests using vStorage to migrate VMs from the old DAE to the new one.   Is his suggestion valid ?  Thanks 
Migrate VMs by using vStorage ?