If you are on a budget, an SAR-1 (Romanian AK) is a great deal for under $300, will shoot EVERY time you pull the trigger, you can shoot inexpensive Wolf 7.62x39 ammo, and you can get new, very sturdy, reliable 30 round mags for $10 a piece.
It's not going to have the accuracy of an AR, but can hit a volleyball sized target at 100 yards, and it will work every time. The sights are kind of big for hitting stuff over 100 yards, as the front sight post will obscure them.
I have an SAR-1, and a Bushmaster. Though the Bushmaster would be my first gun to grab to defend my life, it (and all AR's) are very susceptible to magazines and ammo that aren't quite upto spec.
I had to really learn what it liked in terms of magazines, and ammo (doesn't like PMC), before I could trust my life to it. And the reliability issues are just a bad mag or lot of ammo away, wheras the AK is much more easy going.
Though the AR is my number one gun, if I had to defend my life within a house, I'd grab an AK before an AR. Once a target was over 50 yards a away, the AR starts to be the better choice, IMO.
You won't be dissapointed with an SAR-1, they look like crap, with crude milling marks all over them, and sometimes slightly off center front sights, but function without a hiccup.