I'd say the rear is probably the better way to go.
But I have an Aussie just in the front and I haven't broken any of the stock drive-train or found myself unable to climb nose-first on some of the harder so. cal. trails (course its a different story if I've gotten high-centered). So my needs, thus far, are met with the front only set up.
If you have a rear locker, you may find times when you wish you had a front one and Vice versa. Some might say that there is less of a chance of that with a rear-only versus a front-only. Probably.
However Tires and articulation may influence the traction equation significantly, also.