I have shot both. My old Mini-14 from ~1980 shot as well as a comparable AR. And certianly better/more reliable than what USMC originally got in 'Nam. Yes, I could build, or buy a premium hbar AR15 that would outshoot a Mini-14. But for a service grade AR in the $700 range like the Mini-14 I don't think you will find a name branded quality AR that will outshoot it.
The AR does have the edge, in modularity and customization. Most work on a Mini-14 is gunsmith type, not the plug and play of an AR. Just like the 1911A1 trumps most other pistols in that regard(it is pretty easy to build a custom 1911 from a bare frame).
The Mini does can be customized though
Ruger Mini Ranch Rifle Conversions 14 30 Accurate Accuracy Systems
I like them both..