Some SKS were originally designed to use AK mags, but many sellers have guns which were probably converted and many of these have problems.
Check Gunbroker "SKS" with large mags in the photo. The vast majority of the SKS were only built with the fixed 10-rd mag, but you could never forget to bring the mag (as long as you have loaded stripper clips kept in a bag attached to the gun). A careful, thorough German friend was really embarrassed when he forgot the mag for his AR-15.
In an emergency, it would be easy to forget for an AR, M-1A1, G-3 etc.
If not this correct type of SKS, from my unique perspective, it depends on how much ammo you want/can afford to buy, then the rifle.
Otherwise, as most self-defense issues for civilians appear to happen at a very short range, an AK. Criminals do their best to sneak up close to people, as they almost always use handguns.
This is not Iraq or Afghanistan, where an M-14 is used mostly against much more distant targets, offensive and defensive.
Even after a local disaster, when would a regular civilian here need an offensive capability, except for hunting? After the SKS, AK + ammo are acquired, if you are resolved to shoot further, why not later also buy a Mosin Nagant ($80-130) which uses powerful, cheap ammo, if you don't already have a mid-range rifle?