I've had Yugo and Norinco made SKS rifles, and have never had problems with the extended magazines.
Personally, I would stick with the fixed 10 round magazine unless you feel there is a need to have the higher capacity.
P.S. I'll say it before you get jumped on by the other members on this forum. They are not called clips, they are called magazines. The only thing related to your SKS that is a "clip" is the 10 round stripper clips that can make loading your 10 round fixed magazine a little quicker/easier.