Here in Utah, the matches are put on by UDPL (Utah Defensive Pistol League) which is actually an IDPA club. Great matches. Probably my favorite kind of match so far. The next one is on the 30th, and I'm trying to decide between taking my FAL or my Vepr.
From what I've seen at this local club, there have been a surprising number of AK type guns, followed closely by ARs, and a smattering of M1a's, SKS, & FALs. There have also been Galils (used by one of the best shooters I have ever seen). Not used in the match, but pulled out afterwards are the local SWAT cop's MP5s.
I'm not sure why the AK is so popular, but the last 3 gun I went too was on new years day, and it was dang cold, and snowy. But most of the ARs seemed to run just fine.
The AR has the advantage of low recoil and quick mag changes. Also the match rules required the shooters to start with the safety on. This also gives an advantage to the thumb safety vs. the clacker type.
I haven't noticed very many .308 shooters. Most of the stages required 3 shots on each target, with multiple targets. Of course this is easier to do at time with a .223, 5.45, or 7.62x39 than a .308. (Though in the rules, those of us shooting .308 shouldn't have to shoot the target 3 times, thats why we use .308 to begin with.
For me personally I own a Bushmaster, a FAL, and a Vepr. I haven't had the best experience with reliability in my Bushy. I used the Vepr last time and did pretty good (score wise). But my FAL is my favorite rifle, and the one that I am most likely to use in a real life worst case scenario.
I think that any of the major rifles should work just fine.