Mac OS Extended (Journaled) is your only option given both requirments sbove. FAT32 will work as well if you can accept the 4 GB files sixe limitation. exFAT is not supported.
so if i was to plug the Mac OS Extended formated hard drive into a windows pc, will i be able to access it and work with it as if it was a NTFS formated disk?
Suggest that you test using a few small files to make sure that the Mac formatting will work when the drive is connected directly to a PC. I cannot remember whether it will work that way or not.
I do remember that the PC will be able to connect with no problems when the drive is connected to the AirPort Extreme since I used this type of setup a few years ago.
If you cannot run a few tests first, then you should format the drive in FAT32.
Is exFat supported for the disk sharing in the airport extreme
No, exFAT (aka FAT64) formatted HDDs are not supported by the AirPort when connected by USB for file sharing. As you are probably already aware, the AirPort File Server will only work with drives, formatted in: FAT16/FAT32 or HFS+. Note: Regardless of format, the AirPort will use both the SMB & AFP file sharing protocols to share out the drive to Windows, Linux, or OS X clients.
I want to keep the disk format in a readable form for windows and mac, this a external hard drive that i carry with me sometimes for file sharing.
The only option for this would be FAT32. However, the AirPort works best with HFS+ and you can mount HFS+ volumes on Windows with third-party solutions. One example is: Paragon Software's HFS for Windows