the AR15 is the dominant choice of competitors in every competition where the design is allowed to play.
Only where semiauto is a necessity or an advantage. The AR can be extremely accurate for a semiauto. It also has extensive parts availability, and it's unique among semiautos in the ease with which the barrel can be floated, with no moving stuff running along the barrel.
The DI design is great for accuracy as well as maintenance, since it's the closest thing to a bolt action that isn't a bolt action.
Anyone who says an AR isn't accurate either can't shoot or has a lemon for a rifle.
The magazines are readily available, and it's easy to get good ones for cheap.
That said, from personal experience, I trust the Mini-14 action to go bang, eject, and feed, particularly when neglected, over the AR. The bullet may only go in the general direction the barrel is pointing, but it will go...
That assumes good magazines, and of course that's a BIG fudge...
I guess now Ruger sells mags of various capacities, and I have some, but until recently, I got as many bad-to-mediocre Mini magazines as good ones. Meanwhile, every AR mag I have stuck in the thing has worked perfectly. Unless Ruger gets a big military contract, the Mini won't be able to compete for price, availability and reliability of magazines.
And the way the Ruger mags insert really sucks compared to the STANAG magwell -- another reason for competitive use of the AR. Who wants to fool with the Ruger system under stress?
Competition guns aren't always the best measure of a rifle's field-worthiness either way, though. On the one hand, competitors are seldom far from their range boxes full of tools and lube. On the other, match guns sometimes sacrifice all-out reliability for accuracy, so match guns can be more finicky than their utilitarian counterparts.
One more thing... I think the AR will beat the Mini pretty handily when it comes to longevity. Put 10,000 rounds of 5.56 through both, and I'm betting the AR will be the gun that still works without repairs (as long as it's lubed).
I like my Mini for what it does well (handles like a real carbine, points naturally and goes bang even when neglected). But I sure wouldn't want the US military to be issued the things in Iraq or anything. I like my AR for what it does well: hit the target with accuracy sufficient to make shooting it fun, not overheat in 5 rounds, and offer infinite configuration options, should I want them.