It all depends on how much you have to spend. An AK/SKS is a good choice and an AR-15/M4-A1 is a good choice also. Each gun has their strengths and weaknesses.
Reliability: AK has a slight edge here but not by much. The only real problem with an AR-15 is if you shoot PSP's (Exposed Lead Nose) you'll tend to get lead buildup on the feed ramps and after 100-300 rounds it'll have a tendency to jam. If you shoot FMJ's you'll not have this problem. If both guns are kept clean, the only way either one wouldn't fire 1st shot is because of bad ammo.
Power: The 7.62x39mm is a 123gr. round that exits the muzzle at 2400fps. with 1500ft/lbs. of energy. The 5.56x45mm/.223 Remington is a much more common sporting round so bullet weight varies quite a bit from 40gr.-77gr. so we'll take the standard USA Army 62gr. FMJ load with a Muzzle Velocity of 3100fps. and Initial Energy of 1300ft/lbs. The only real difference between them is that the NATO round is half the weight, but makes up for that with increased velocity. The important thing is that the 5.56mm when striking soft tissue at high velocity will as quoted here,
As with all spitzer shaped projectiles it is prone to yaw in soft tissue. However, at impact velocities above roughly 2,700 ft/s (820 m/s), it will yaw and then fragment at the cannelure (the groove around the cylinder of the bullet). The fragments disperse through the flesh causing much more internal injury. The effectiveness of fragmentation seems to impart much greater damage to tissue than bullet dimensions and velocities would suggest.http://www.nationmaster.com/encyclopedia/5.56-x-45-mm-NATO
But the drawback is if you don't strike your target with the required velocity fragmentation won't occur. Another drawback to the light weight is that the bullet is easily deflected by cover, and is also less accurate in high winds. Neither round will reliably penetrate Class IIIa Body Armor.
Accuracy: Neither weapon was designed to shoot more than 300yards. So this isn't as one sided as people would make it out to be in favor of the AR-15. With either gun you can reliably hit the center of mass at 300yards with a properly sighted in weapon. Though I would have to say the AR-15 is more accurate due to the fact that the round is flatter shooting, 22in. of drop at 300 yards compared to 44in.
For me I would take an AR-15, just for the fact that in close the 5.56mm does have more stopping power because of the design of the bullet and it's tendency to yaw and fragment. The AK-74 that replaced the AK-47 shoots a 5.45x39mm round, and takes the fragmentation a step further by having an air bubble in the nose of the round just to increase this effect. In the end though you can't go wrong either way, it's up to which gun you feel comfortable trusting your life to.
Just a note check out the link I posted because there is a slight difference between a military 5.56 NATO and a .223 Rem. in how the chambers are milled. The rounds are interchangeable in a pinch, but not recommended for constant use.
