Are you set up well for the firearms you have now?
Plenty of ammo, magazines, parts, etc?
If not, I'd take care of that first.
You could just put a quality AR on layaway.
This would allow you to pay over time, and avoid the whole credit card thing.
While you are making payments on that AR, start acquiring quality mags and ammo.
Since you don't have the rifle in hand, you'll have time to shop around for good deals.
At the prices they're at now, I have a hard time thinking mags will get any cheaper.
When you have that one, well thought out and kitted out rifle in your grubby little paws
, do it again.
You could split the difference.
Get a CC with 0% interest for so many months, and purchase a solid AR, along with SOME mags and ammo.
Pay that mother off!
This gets you something in your hands now, in case the North Korean paratroopers drop tomorrow.
Add more ammo, mags and spares as time and $ allows.
Now that you have that one rifle with its accessories and your CC is paid off, find another AR at your local gun shop
and put it on layaway.
As far as a SHTF battery goes, I'd stick to one platform or the other.
Arguments can be made for being able to use different kinds of ammo, but IMHO, that benefit is more theoretical than anything.
You're going to have two different kinds of ammo to stock, along with mags, pouches, spare parts, etc.
If you're going to be proficient in the use of both, your training time and $ is doubled.
Once your SHTF firearms goals are met, you can always get your AKs later.
Contrary to arfcom conventional wisdom, enjoyment is a valid reason for firearms ownership.
Good luck.