The Auburn is good if you never want to think about whether the differential should be locked- it does it by itself depending on road conditions, but it is technically not as good as the spartan type lockers off road. on road its a very well behaved unit.
The spartans are always locked under load, hence the need for free wheeling hubs if installed in the front end.
The price difference will be about $500 as you do need the gear kit with the Auburn.
Search this site for references to the spartan kit for about $230