If it's just for predator protection issues, I'd lean towards an 870 12 guage, with marine finish & slugs.
http://www.remington.com/products/fi...ne-magnum.aspx
The Mossberg 500 JIC (Just in case....) in marine would be my second choice.
http://www.mossberg.com/products/default.asp?id=28
It will only be used close up, and it can handle the moisture fairly well, and a lot quicker to rack than work a bolt. Just my own preference. If you're set on the M70, either cartridge you mentioned will get the job done as well.