SKS is going to be cheaper. It's also going to be heavier. I've seen a few Yugos that were unreliable, but overall its a very solid rifle. The 10rd capacity can be annoying, but its reliable. If you get tired of loading it with stripper clips you can get a detachable magazine that will never work right. There are lots of accessories for the SKS. Folding stock versions make it lighter, but also mess up the balance of it, IMO.
The detachable magazine is the main difference between the two. AK rifles are lighter, better balanced, cooler looking, and more fun, but way more expensive. I haven't been around them enough to know about reliability, but have never heard of any problems with an AK. Most that you find are romanian, which seem to be the most cheaply made, so if you find one thats not, whether its Yugo, or chinese, or whatever, go for it. You can also get the russian made Saiga, which is a very high quality AK for the price (and comes in a huge variety of calibers, from 7.62x39 to .308 to .410 to 12ga, but most don't have a pistol grip.
Accessories for both the SKS and AK are plentiful. If you have the money, the AK is a better rifle. If you are on a budget, an SKS will shoot something just as dead and is never a bad thing to have. If you like having guns like I do, get them both. Both will be fun, reliable, and innacurate.
Thats everything I know about the two.