There are important differences between them (explained below). If you are unsure which version to use, you should continue to use the standard 32-bit version.
Do not use both: You should not have both the 32-bit and 64-bit versions installed at the same time on the same computer. Having both versions installed will cause issues.
Migrating: To switch from one version to another you should export your profiles, uninstall the old version, install the new version and then import your profiles.
Windows: The 32-bit version can be used on 32-bit and 64-bit versions of Windows. The 64-bit version can only be used on 64-bit versions of Windows. Refer to the Microsoft documentation to see which version of Windows you are using.
Compatibility: The 32-bit version is compatible with older versions of SyncBack. This means, if you are upgrading from V7 or earlier, then there will be no issues. The 64-bit version of SyncBackPro cannot use VBScript scripting. You must convert your VBS scripts to the new Basic scripting language.
File System: When a 32-bit program is used on 64-bit versions of Windows, then Windows interferes with file system calls to help with compatibility. This means that 32-bit versions of SyncBack will see different files in some Windows system folders. If you are not copying system files, then this should not be a problem. If you are, then you should check your profiles file and folder selections to make sure the correct files are being copied.
Environment Variables: If you use Windows environment variables, e.g. for a profiles destination, then note that some Windows environment variables will have different values depending on if it's 32-bit or 64-bit Windows. Check the Windows documentation.
Open/Locked File Copying (VSS): If you use the 64-bit version with Windows Server 2003, or 64-bit Windows XP (which is not supported), then open/locked file copying will not work. Windows XP is not supported.
Scheduling: The Windows Task Schedules, to run your profiles, are stored in a different folders in the task scheduler. This should not be a problem because SyncBack will import the schedule into the correct task scheduler folder.
Benefits: The major benefit to using the 64-bit version is that it has access to all the memory on your system. This may be important to you when using some cloud services, e.g. Dropbox, and high levels of compression.
Downloading: See the downloads page for links to the 32-bit and 64-bit versions of SyncBackPro and SyncBackSE.