If you format it as ExFAT, you can use it with either Windows or Mac, but that file format pays a speed price that I find to be too much. The way I do it is by formatting for Mac as HFS+ (aka, Mac OS Extended, Journaled) since I mostly use the Mac; then I use the Paragon HFS+ driver for use on Windows. You could also format as NTFS and get a NTFS driver for Mac. macOS can read NTFS drives without a driver, but not write to them. DO NOT format as APFS (the new macOS file format). The Windows drivers for APFS are still read only.