Probably the Mini
The Mini-14 will be functioning long after the AR is stopped by sand, mud, grit or loss of lubrication. Having kept firearms on boats I found all steels react poorly to salt water, including stainless and chromed. Either the Mini-14 or the AR will require rust preventing sprays to keep from becoming corroded junk. I used to spray the firearms weekly on the boat. Ammunition gets verdigris quickly in the salt water environment also; this influencing its ability to chamber. Again, the Mini-14 will chamber dirty ammo that will stop the AR. My choice would be the Mini, but instead of the .223 I would go for the 7.62x39 Mini-30. The larger bore will stay free of corrosion longer due to capillary action of incidental water such as spray being far less likely in the larger bore.