The thing is, like the AK the Mini-14 does not need much maintenance. It's a simple short range carbine.
The AR is more if a 'experts' gun. It requires more knowledge and maintenance BUT if one knows the weapon it is miles ahead of the AK or Mini-14 (and I say that cause I own the AR, AK, and Mini-14! I fact I've owned several.)
For a local police department the Mini-14 will do fine. 100 yard shots are rally rare but the Mini can handle such.
The AR, when properly set up, is a wonderful machine. Capable of hits way way out. But, like I said, it takes someone with more than just a bit of knowledge about the weapon and how to keep it running.
Oh, and I do prefer the AR, if you know what you are doing and can shoot.