You should get the one that is easiest to set up.
Avoid things like auto backup, auto picture optimizer, auto media convertor, they mostly only work "automatic" when a win soft has been installed.
Try to find one that is powered by the USB cable only (power saving and power independant), these normally have all their needs in their firmware.
I have a StoreJet Transcend 500GB (2.5" and comes in a classy black slim case) and it runs perfectly both on the X1000 and as a NAS on my router.
Happy searching