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This depends on how you intend on using your hard drive. Mac OS Extended Journaled is a Mac-only configuration that will allow you to use Time Machine and the iPhoto library with your unit, but does not work in Windows. On the other hand exFAT works on both Windows and Mac for file storage, but will not allow access to some Mac-only or Windows-only advanced features.
You can format your hard drive as many times as needed to better suit your needs. However, the hard drive will be erased every time it’s done.
