State of NTFS Support at OSX 10.9.5
I have found these articles:

http://macdrug.com/free-ntfs-read-and-write-solutions-in-mac-os-x-el-capitan-or-older/

http://www.makeuseof.com/tag/write-ntfs-drives-el-capitan-free/

(pretty much identical)

If you want to reformat the external HDD I guess that the native Apple filesystem would be the most reliable choice, unless you have to use it with other devices... in this case I think FAT32 could be quite "universal".