You can use either. Most of the one's I have sold have been the 1314 but either will work.
Here is a good explanation of how it works. This is for the 200 amp versions but they work the same.
"Yesterday I received my replacement sure power. They actually sent me the 1315-200 instead of the 1314-200. All that means is that this unit will connect both batteries as long as the main or aux see a voltage over 13.2V. For example, the unit will connect with the engine running (alternator is providing the charge) or if I connect a battery charger to the spare battery. The 1314-200 will only connect if the main battery sees a voltage over 13.2V. They call it bi directional (1315-XXX) unit as opposed to a uni-directional (1314-XXX) unit. Hope this makes sense.)