I took my Russian SKS and my CZ 858 to the range yesterday and must say accuracy wise they are no different. Both will shoot 3"-4" at 100 yards off a crappy rest, open sights, with norinco silver box.
I figure that these two rifles aren't really in the same class as one is a "battle rifle carbine" that shoots a short round round while the other is an "assault rifle".
I know some people cringe when the word assault rifle is used but I don't know of an acceptable replacement word that describes a select fire, high capacity removeable magazine, low power rifle round. I guess you can call the Cz858 a neutered assault rifle since it's not select fire and has a pinned magazine.
I think if you want a rifle that is more effective in say a 3 gun shoot then the Cz is the way to go. You have more options loading wise (magazines or stripper clips) plus it comes right out of the box with a pistol grip and a folding stock (if desired). It's also over a pound lighter and is shorter than an SKS and that can make all the difference to some people. You could of course get yourself an SKS-D and add a bunch of goodies with the money you saved like pistol grip stocks with a proper bedding job, trigger job, and nice aperture sights. You'd end up with something very similar to a Cz 858 minus a bolt hold open, plus an extra two pounds and 4" of extra length.
If you're the type of person who wants something to shoot beer cans with or take to the range as a cheap shooting fun gun that's more manly than a 22 rimfire then the classic SKS is the way to go since it's cheaper (even a nice Yugo or even a Russian) and it will do just about everything a Cz 858 will do on the range or in camp. A norinco will also be fine, you might have to go through one or two to get a good one then put some elbow grease into bedding and trigger jobs to get it to shoot straight.
There's also the look cool factor, if you just plain don't like the look of a gun, or if it feels uncomfortable, then you won't shoot it! You'll always find an excuse not to bring it along to the range. It's better to spend more money on something you will shoot, then save some cash and then leave it in your safe.