SnapDrive 6.2.1 for Windows is an update to SnapDrive 6.2 and includes fixes for the following bugs:
Bug ID 393518: SnapDrive 6.2 fails to create RDM LUNs with ESX iSCSI when FCP license is not set on the filer. For more information, go to Bug Tools on the NOW site at http://now.netapp.com and search for bug ID 393518.
Bug ID 395057: SMHV does not work on Japanese Windows OS
Bug ID 397424: SDW: LUN restore fails IOCTL_DISK_SET_DRIVE_LAYOUT_EX on reversed SnapMirror destination
Bug ID 397462: Windows 2008R2 120s Volume Arrival Timeout when connecting to LUN Clone on the same host
Bug ID 397866: Snapshot mounts are very slow.
Bug ID 408511: ESX iSCSI RDM igroup has initiators from other hosts. SnapDrive does not support this configuration.
Bug 424071: SDW Workaround: SME remote verify fails on SM dst with SDW ZAPI snap-list-info response volume offline
Bug 426196: SnapDrive for Windows creates multiple FlexClone volumes for LUNs on same Snapshot copy
Bug 440822: The SnapDrive service in an ESX VM takes too long to start up in large ESX environments
SnapDrive 6.2.1 for Windows continues to support the following major new and enhanced features introduced in SnapDrive 6.2 for Windows:
Support for dynamic addition and removal of Hyper-V®; pass-through disks using Windows Server 2008 R2
Support for Microsoft® cluster shared volumes using Windows Server 2008 R2
Support for restoring data at the file level
Support for RBAC with Operations Manager
Support for RDM LUNs using iSCSI hardware initiators and VMware® ESX iSCSI initiators
Support for creating FC RDM LUNs across two virtual Microsoft cluster machines using Windows Server 2003 and Windows Server 2008 systems
Support for IPv6 with ESX 4.0
SnapDrive 6.2.1 currently has the following limitations:
iSCSI RDM LUNs are not supported across two physical Microsoft cluster machines.
Customers using SnapDrive 5.0 and earlier on Windows Server 2003 IA64 systems must upgrade to Windows Server 2008 on IA64 systems before they can use SnapDrive 6.2.1.
A LUN managed by SnapDrive cannot be configured as a "dynamic" disk (a storage device that is divided into volumes rather than partitions); it can serve only as a "basic" disk (a storage device for host-side application data).
A LUN cannot be configured as an extended partition; SnapDrive supports only a single, primary partition on a LUN.
A LUN created with FilerView® or at the storage system command line can be managed only if certain steps (documented in the SnapDrive for Windows Installation and Administration Guide) are taken to prepare these disks for SnapDrive.
Conversion of a LUN with an MBR partition to a GPT partition is not supported.
Permanent clones are not supported.
Snapshot™ copies that are created from the storage system console are not supported; because this can lead to inconsistencies within the NTFS file system, only SnapDrive should be used to create Snapshot copies of LUNs.
Volume-level SnapVault® configurations are not supported (however, qtree SnapVault configurations are supported).
Considerations when using Microsoft iSCSI Software Initiator include the following:
In an MSCS configuration that uses volume mountpoints, install Microsoft hotfix 925677 on all cluster nodes if you encounter the issue described in Knowledge Base article KB925677.
In a noncluster configuration, the Exchange® service and the SQL service must be configured with a dependency on the iSCSI Software Initiator service.
SDW 6.3 was just release and is still in "First Customer Shipped" status as opposed to 6.2.1 which is official "GA" .
There would be no reason to upgrade to 6.3 p1 unless you were looking for a specific burt fix or functionality.
Major difference between 6.3 would be the integration with VSC (aka SMVI) , SMSQL, and NFS datastores.
SnapDrive 6.3 for Windows includes the following major new and enhanced features:
Support for performing Snapshot copy operations on virtual machine disks (VMDKs) in an NFS or VMFS datastore, which requires Virtual Storage Console 2.0.1 or later. Please check the availability of Virtual Storage Console 2.0.1 and install it before using this feature.
Support for space reclamation when using VMDKs in an NFS datastore
Support for DataFabric Manager caching with RBAC
Automatic download of the storage system access control (storacl.exe) tool as part of your SnapDrive 6.3 for Windows installation
Bug fixes For more information, see the SnapDrive 6.3 for Windows Release Notes.
If you need any of this then you would need to upgrade, otherwise I would stick with 6.2.1 for the burt fixes available.