I have four SKS rifles, and after much fooling around, they all have the original 10 round mags. If you want the 30 round capacity, I heartily suggest going with an AK, which was designed for them. They are far more likely to be reliable than an SKS with aftermarket mag.
Now, I'm sure there are people who have 30-rounders on their SKSs who will say they are fine. My experiences have been otherwise. That's all. The original magazines are very good. With practice, you can load them from clips very quickly, and the ammo on clips is much lighter than an equal number of rounds carried in steel magazines.
It seems the Garand made a big impression on the Ruskys, and for good reason.
Also, the SKS bolt locks around the magazine's feed lips when in battery,
unless it has had its bolt customized by cutting away at the bottom of it, the SKS magazine can be removed only if the bolt is locked back, and you can't lock the bolt back unless your magazine is empty, making for problematic inspection of your load, or for "tactical reloads". You can do it by holding the bolt back with one hand and removing the mag with the other, but its a bit of a stunt. Furthermore, I never was able to load a detachable SKS mag from a stripper clip-- it always seemed to release the bolt in the process, endangering the thumb. This almost never happened with the original, serial number-matching, 10 round mag.
Detachable box magazines are best used in rifles that were originally designed for them.
