I own both. Ruger Mini 14, and Remington 870 shotgun. Cant make a complaint about eather. The P/D that I retired from used the A/C 556, and semi auto Mini's for many years, and only within the last few years are transitioning to the A/R platform. The Mini 14 is my go-to gun in any case a rifle becomes a need. This includeds self defence. The 870 shotgun as everyone says has long since paid it's dues. It's capable of feeding you, protecting you, and wont be a drain on your wallet.
I'm curious... What kind of work would require that you become familiar with these weapons. Were it my choice I'd allow police officers, and deputies the option of using a mini 14 or an A/R as a patrol rifle.