From an article linked at AR15.com...
http://www.ar15.com/content/swat/200203-AR-15_vs_Mini-14.pdf
To check the worst-case scenario, at
one point we dropped about half a handful
of loose dirt into the magazine well
of each rifle and then shook each of
them to distribute the dirt. After running
a cleaning patch down the bore of each
rifle to make sure the barrel was clean
(no sense in being completely stupid and
having a rifle blow up in your face) we
attempted to fire a 20-round magazine
rapidly through each. The Mini-14 ran
without a malfunction. Much to my surprise,
my usually malfunction-free AR
burped on the third round, and did not go
fully into battery.
...this had a lot to do with my choosing the mini over an AR, but anyone who has shot both will concede that a good AR is more accurate. Mini's can downright blow chunks accuracy-wise (however, they're at least as accurate as the AK's--another rifle designed not for tack driving, but functioning under harsh conditions).
If someone is well experienced with both rifles, he'll admit that the mini is more reliable than the AR. What the mini gives up in accuracy to the AR, it gains that much back in reliability. That'll put a burr in the bloomers of many here, but it's nonetheless the truth.
Dan