WD Passport or other External for both OS X + WIN8.1

WD Passport is as good a drive as any.
If you format for Windows (NTFS) or both (exFAT) you can do anything except TimeMachine backup. The latter needs Mac-specific formatting (HFS+).
Comprehensive coverage of the topic:
https://forums.macrumors.com/showthread.php?p=18523218#post18523218

varatep said:

so it is not possible to backup my os x data at the same time as being able to store files into the passport from my windows 8.1 bootcamped?

Click to expand...

You'll need at least one HFS+ partition, and one NTFS or exFAT partition. exFAT works both with Windows and OS X, but like priitv8 said, Time Machine only supports HFS+. With the Boot Camp drivers Windows can only read HFS+ partitions, but not write to them, and OS X can only read NTFS partitions. The post priitv8 linked to also contains some tools to enable NTFS write support in OS X, and HFS+ support in Windows.

So you could create an HFS+ partition, that would be only used by Time Machine, and then an exFAT partition that can be accessed both from Windows and OS X.

Fully backing up OS X with Time Machine will still require you to boot to OS X, and the same mostly goes for Windows. There is a way to backup the Windows partition from OS X, but it's not free: Winclone. If you have a backup solution in Windows, it might also be able to backup files from the HFS+ partition, but it can't make a full backup of the whole system.