The WASR is widely considered bottom of the line. I have one, and it hasn't given me a lick of trouble, but that seems to be fairly uncommon. WASR-10s were designed for single stack magazines, and are converted to double by Century. It appears as though they use a dremel to trim the mag well. Generally, they take too much off and it makes the magazines wabble (sometimes severely). I'd DEFINITELY inspect that rifle before I bought one. You're taking some fairly high risks by buying it online. They're about as accurate as any other AK I've fired.
The Yugo is built on an RPK style receiver, which is 1.6mm instead of the standard 1mm. Because of this extra thickness, it's much more durable (but also a bit heavier). Build quality definitely goes to the Yugo. One thing to consider is that it does NOT have a chrome lined barrel. Accuracy definitely goes to the Yugo though.
I'm having some problems with my Yugo, but I think my case is extremely rare. I can't find anything online that parallels my problem, and almost everyone I've talked to about this rifle is very pleased with theirs. One of my friends has one (convinced me to buy one) and it's a great firearm. He loves that rifle.
If I had to make the choice again (even though I'm having some problems with my Yugo) I'd take the Yugo.
That's my opinion anyway :]