For easy sharing a drive between a PC and Mac use FAT-32. But keep in mind that FAT-32 is limited to a single file size of 4 Gb. If you plan on saving files that are larger than 4 Gb use NTFS. However, in order for a Mac to read and write to NTFS you'll need third party software like the Paragon NTFS driver.
Paragon NTFS for MAC OS X - full read and write access to Windows NTFS partitions under Mac OS X
Regards.