I have both a Mini-14 and a AR-15. Both of them shoot very good IMO. 

 I would stay away from the Mini-14 Ranch rifle and spring for the Target Ranch Rifle like I have, it has a bullet tuner which makes it shoot much better than just the plain Ranch Rifles.  Also, look at the A-Team.  They used just plain Ranch Rifles and shot thousands of rounds of ammo and killed very few people.  They could have done alot more damage with Target Ranch Rifles The downside of the Target Ranch is that it's pretty heavy.

Now the AR-15 is the more expensive of the two and is lighter (in some cases) so it is more comfortable to carry and shoot. 

Either one would be just fine really.