Unless you are just looking to fill up the safe again I would only encourage someone in your position to purchase one serious combat rifle. That way, you only have to train with one system. The AK and AR share nothing in common and require changes in reloading techniques, magazine carriers, and ergonomics. In a serious situation you revert to your training and unless you train a lot with a lot of different weapons you may fumble at the worst possible time.
Again, I would choose just one rifle. I know this is the retro forum so one of my first picks would be something along the lines of the Colt SP1. There are still vast quantities of 55 grain ammo out there, plenty of magazines, lots of spare parts for support, and it is a very reliable 5.56mm rifle with a proven history of quality. The Colt SP1 is a good combat rifle as is.
Now, if you wanted to go modern but stay stock there are a few other choices, though I won't name them all. SIG offers the SIG 556. It is a piston operated rifle which holds an advantage over direct impingement rifles like the AR15. The system runs cooler and doesn't blow carbon back into the bolt group or firing mechanism. It uses AR15 magazines but has a faster twist for the 62 grain ammo, though it will still shoot 55 grain ammo just fine.
A second option if you want something that looks really cool would be the Micro Tech clone of the Steyr AUG. It uses the same magazine as the Austrian AUG and has the faster twist for heavier bullets. It's very compact as it is a bullpup.
Another bullpup is the FN FS2000. This rifle uses AR15 magazines and has a faster twist for heavier bullets. It is unique in that it does not eject empty cartridge casings like most other guns. Instead, it controls the spent casing and pushes it into an ejection tube that lies on top of the barrel. The cases are eventually pushed out the end well away from the shooters face.
Bushmaster will soon release their version of the MAGPUL rifle which is supposed to be as near perfect as a combat weapon can get.
Another good rifle would be the XCR. It is a very simple, rugged, no nonsense 5.56mm rifle that also uses AR15 magazines.
The M96 is another interesting choice. The stamped sheet metal receiver is made of stainless which is colored black. Very corrosion resistant. Also piston operated much like the original Stoner from which it was copied.
I'm sure I left out a few fine choices for a 5.56mm but those are a few highlights that would be capable of the job.
Depending on the terrain in your area you may or may not be better off with the 5.56mm as your first choice. Some places would be well suited to the 7.62x39mm or even the 7.62x51mm. The latter would depend on your ability to carry the weight of enough ammunition. The X39 would be much lighter and the 5.56 would be even lighter.
Other considerations would be targets. Would the targets be armored, hardened, or just simple zombies? Ammo selection must be carefully considered in order to be the most effective.
Here's what I would really consider doing if I were in your situation. I would buy one good rifle that cost between $1000 and $2000. Then I would invest about $2000 in ammunition. A rifle without ammunition is a club. Then I would find a decent training course for less than $1000 that would teach me how to use that rifle and survive a gunfight. A rifleman without training is a danger to himself and others. If you are serious about investing in a rifle that could be used for combat, you should train for combat.
While you are training you will find out that some techniques work better than others. Take the best techniques that work for you and practice those often. You will also find out along the way that some gear works better than others. Invest in quality gear, cheap gear is just cheap gear.
If you want to be a warrior you must train like a warrior. Don't half ass it.