I was just asking myself this question a number of weeks ago. The responses I got to the threads I started (on other boards) didn't help much, since it quickly became clear that everybody has their own opinion on the matter. There isn't much of a consensus. That means I have to drag out the old "try them both and pick the one you like better" clichè.
That said:
I ended up with an AK, because that's what I really "wanted." The SKS came into the equation because it was cheaper, and I saw an opportunity to get a similar rifle and save some bucks in the process. If a typical entry-level SKS (Yugo, usually) can be had for about $200 (I have a C&R) and the typical entry-level AK is about twice that, the difference in price equates to a good bit of ammo. In the end, though, I thought the AK was simply more fun to shoot (pistol grip and better trigger contributed strongly to that) so that's what I bought.
If you're simply looking at the SKS as a "poor man's AK," there's a good chance you won't be happy with it and will start wishing you had just spent the extra cash up front (that's what I was afraid of, and what ultimately pushed me to grab the AK first). They're different guns. They handle differently, and only really carry a passing resemblance to one another (mostly in terms of the forward hand guard and gas tube placement). I see a lot of people who buy an SKS and then start tacking tacticool stocks and such to them, presumably in an attempt to get an AK clone for less money. It really doesn't work that way.
A couple points beyond the obvious (e.g. appearance):
-The SKS has a nominal advantage in accuracy thanks to a longer barrel and sight radius (though don't forget that the 7.62x39 is a ballistically challenged cartridge past 200m anyhow).
-The SKS also has a bolt hold (at least the ones I've shot do), which is a plus.
-Certain AK variants (e.g. Saiga, Romanian) have the standard Soviet side rail mount for optics. This means that installing a Russian style scope or weaver mount is as easy as sliding it onto the rail.
-You can get all sorts of AK mags for $10 apiece. The SKS generally doesn't like anything aside from the factory box mag, so you're stuck with that unless you want to modify or replace the stock and spend a lot of money for custom fitted mags like the ones Tapco offers. There are a few SKS models (like the Chinese SKS-M) that are designed to take standard AK mags, but they're not the cheapest or easiest to find.
-Both the Norinco and Yugo SKSes that I shot had pretty bad triggers.
Another thing to consider: virtually all of the SKS rifles you'll come across these days are milsurps (I wouldn't be surprised to hear they're still making new ones in China, but we won't get those thanks to the import ban). If you have a C&R, that could be a good thing, but it could also be pretty bad. Now, since the SKS was superseded by the AK in front line service pretty quickly, there are a lot of guns that didn't see much service (some were simply packed away and never issued). If you can get an "unissued" or "excellent" SKS, that's great, but you're probably going to pay $250+ for it. You might have trouble getting them for a good price locally, too, because some less scrupulous dealers try to pass them off as AK derivatives and mark up the price accordingly. Otherwise, you're playing the milsurp game, which means you have to start worrying about things like rust, finish and bore condition - for example, I've read of people who buy Yugos only to find that they aren't cycling well because the gas valve is corroded or blocked. Also, those same Yugos don't have chrome lined bores and were used to shoot corrosive Combloc ammo, so you might see frosting or pitting in some of them. By the time you're done, you could end up spending close to $300 for the right gun.
That's much less of an issue with the AKs, because those are still built and sold as factory new rifles here and abroad today. It isn't uncommon to find recycled parts in "new" rifles (especially in kit builds), but at least you'll probably be getting things like a new bore, receiver and trigger.