MS-DOS (Fat) or ExFAT??

I want to give a spare hard drive to a teen in our church to use with her Windows laptop as a backup drive. The drive is currently formatted for the Mac OS, and it has been years since I used anything under Windows. Mac Disk Utility gives me two choices to format the drive for Windows (I think) - MS-DOS (FAT) or ExFAT. The teen's computer will be running either the newest version of Windows, or perhaps one version older. What format should I use for this drive? Thank you!