I understand that, my point was the OP wasn't asking about top of the range £400 tablets, he was asking whether the Nexus 9 was worth the extra £50 over the Shield.
General responses to that seems to be no unless you actually want the bigger, better screen, and faster updates.
Obviously pointing out the other options is valid, and that other options will be available shortly. But the Pixel C isn't going to offer him anything more than the cheaper tablets, other than maybe a bit more future proofing and a slightly bigger screen. That isn't worth £250 more than the Shield provided the screen size is acceptable.