SKS. Sorry.
Yeah, I'd go with the SKS.
Before you blackstick people come out of your chair, let's be fair here.
The question was "defense" not "assault rifle." SKS wins.
"Defense" vs "Assault Rifle" to me means you might have to use it in your house or yard ONCE againse a SMALL number (maybe 1) bad folk, not participate in an urban scenario like Baghdad.
I carried an M16 for three years in the Army. The onlyl thing I liked about it was I had the M203 40mm grenade launcher on mine so I could count on ONE weapon. I could hit very well with my M16, no sweat, but to me, life begins at thirty (caliber!) and the SKS wins. I own three, and have shot 'em for 20-odd years. No, they're NOT pretty, they're NOT as tight and accurate as an M16 - they're just NOT. However, they're reliable as dirt, and if the magazine bothers you, you can get a 20-round fixed mag that's very good and reliable. If you can't do what you want for "defense" with 20 rounds of ammo, you don't need to be here. That kinda also touches the ammo deal - if you can't stockpile enough ammo (like a couple of boxes) you shouldn't be worrying about availability. Again - original question - we're talking defense here, not some SHTF broken planet scenario where you somehow think the National Guard or somebody is going to be air dropping ammo to you in M16 mags. Neither round is legendary for lethality, but let's stick with the question the way it was asked.
I can keep strangers off my property with my SKS, thanks. Oh, by the way - they're reasonably effective with a red dot sight mounted, kinda like a beefy Ruger 10/22. I know, if somebody's beating on your front door in the middle of the night, you won't need to turn it on anyway - you'll be point shooting.
Okay. I'm gonna jump off my soapbox here so you AR15 lovers can start beating on me