Apart from dragging your AR through a muddy swamp, landing on it after jumping out of an airplane, or firing it underwater the AR is a reliable carbine IF it is lubricated.
A Colt 6520/6720 with a MagPul PMAG magazine is about as compact, lightweight, and reliable a weapon as a civilian needs. The aluminum USGI magazines that come with the carbine are range use only, IMO.
If you frequently find yourself in mud, fine sand, snow and ice, or underwater then perhaps the AK is a better option.
The ergonomics, balance, weight, accuracy, availability of parts, and options for optics and weapon lights all favor the AR. The AK is more reliable in adverse conditions, that's it. The AK-47 and AKM fires a more powerful cartridge than the AR-15. The AK-74 fires a slightly less powerful cartridge than the AR-15.
Arsenal makes solid, reliable AK's built to Russian or Bulgarian military specifications. Arsenal guns are new manufacture weapons with all new parts built by Russians and Bulgarians, not used parts kits with new receivers built in ways that may vary considerably from original design specifications.
That said, parts kits are not necessarily all beat up and worn out. Some will be perfectly serviceable, but you'd only know that by eyeballing all the parts and having the knowledge to discern unserviceable parts from serviceable ones. If you can afford to have a gunsmith like Jim Fuller build an AK for you with a parts kit in good condition, you'll have a properly built, reliable, and accurate infantry weapon. Purchase some Bulgarian Circle 10 steel reinforced polymer magazines or steel European or Chinese magazines (if you can't obtain reliable polymer magazines) and you're GTG.
The 5.45 ammo is the cheapest, but 7.62 ammo hits harder and is still pretty cheap. The brass cased 5.56 ammo is considerably more expensive, in normal times, than 5.45 or 7.62.
I prefer steel cased Golden Tiger for AK's. From experience, the Golden Tiger seems to burn cleaner and is more accurate than Wolf or Tula in my AK's. The Golden Tiger ammunition has both a bullet and primer sealant applied, so it's ideal for storage. I have tried Winchester white box cartridges in my AK's, but they are slightly less accurate than Golden Tiger and cost considerably more. It's odd, but that's what I've observed from using Wolf, Tula, Golden Tiger, and Winchester.
I prefer Federal M193/XM193 55 grain FMJ for AR's. That is what the AR's were originally designed to use. The M855/M855A1 62 grain FMJ enhanced penetration or actual AP ammunition is not suitable for home defense and more expensive than M193.
