Surepower 1314 vs 1315?

Just wanted to find out what us the major difference between the 2. I know one is bi-directional but is there a benefit of having one over the other? I am planning on using one for my 2 battery set up and wanted to find out which one is better to use. Seems like most people use the 1314.
Thanks

Your alternator only charges one of the batteries so the 1314 is fine.

Might only be beneficial if you for some reason you hook up a battery maintainer to the other battery...otherwise just put it on the main battery.

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.)

Makes sense, but is there an advantage of having one over the other? A guy I know has a 1315 he wants to sell cheap since he wants the 1314. Just don't want to get stuck with something that may cause issues.