SKS rifles an Armorer point of view
While I can't comment on the idiosyncrasies of the AR-15. I feel that I can provide a good deal of information on the SKS. I have repaired over 500 SKS rifles this year so I can give you an idea of what can go wrong with them perhaps giving you a good idea of what spare parts you might need.
All of the rifles I have repaired have seen hard usage in foreign service much of which is due to neglect and or abuse despite that it has given me clues as to what parts are the most likely to fail.
While the the SKS is a fairly simple rifle and well made it has a few flaws. The first and foremost is the trigger group. The trigger group alone has 20 parts. And I have seen most of them damaged or broken in some way. Most common types of damage are broken off safety levers, weak safety springs, broken hammer springs, broken disconector bars, broken hammers, worn out trigger guard latch retaining holes in the trigger group or bent trigger guard latches.
So rule #1 for the SKS is buy a complete spare trigger group. The small parts in the SKS trigger group could easily be lost in the field so its unwise to attempt to disassemble it. Its much better to replace it completely. Also test it in your rifle to make sure it works properly before finding out when you really need it, that it doesn't work.
Another common broken part in the SKS is the wooden stock. I have seen all descriptions of cracks, breaks, splits, chips , fractures and holes in these relatively thin stocks and I would recommend replacing the original wood stock with a synthetic one. I cannot suggest a good commercial synthetic stock as I have no personal experience with them, but I know they will not break like the wood ones do.
As far as the bolt group is concerned I have occasionally seen broken firing pins . More often a firing pin will be found rusted or stuck in the forward position by crud, which is bad. Always check to see that the SKS's firing pin is free and moves easily in and out, check this by pressing on it with a small punch.
I have seen weak and broken recoil springs but not too commonly. Occasionally the recoil retainer will be broken or missing causing the recoil; spring assembly to fly apart when the receiver cover is removed.
The take down latch lever is far too often to be found broken off and can only be manipulated using pliers or vice grips. Be gentle with the take down latch.
Box magazine assembly's are found to be damaged from time to time but more often the follower or it lever are bent. Often the bolt hold open tab is broken off the follower and wont actuate it. I have found the very small spring in the bolt hold open to be weak, broken, missing or so completely full of cosmolene and grime that it won't work. Try using solvent on it first to see if its just gummed up, this often frees it but many times the spring is broken or missing and will have to be replaced. The bolt hold open spring is very small about the size of that found in a ball point pen and is easily lost.
As far as the gas system goes it is usually pretty good most of its parts are very stout. I have seen budged, cracked and rusted out gas piston tubes however and weak or missing tappet spring and tappets. Many times the piston will be frozen by rust or cosmolene and need to be cleaned so that it can move freely. Occasionally a piston will be so worn or pitted as well as its tube that it won't have sufficient pressure to work. The easiest way to check this is to place your thumb over the piston tube and let the piston fall downward if you feel suction on your thumb you probably have suffient gas pressure to cycle the action. On the Yugoslavian SKS there is an aditional gas cut off valve which can be turned on and off for grenade launching many times these are rusted or gas cut and must be replaced. Also the button and spring which actuates them can be broken off or missing.
Rifles without chrome lined bores are often very dark and or pitted. Front site hoods are often crushed or broken, rear sight springs are found broken from time to time but rarely.
Given what I have seen I would suggest an SKS with a chrome lined bore either Chinese, Russian, Romanian (being the easiest to find). A good synthetic stock. A Spare trigger group, firing pin, extractor, extractor spring, front site post, Recoil spring assembly.
Also a cleaning rod, complete SKS butt trap cleaning kit with the addition of an AK front site adjustment tool. And a good web or leather sling.
Brother in Arms