I have the 30 round and 20 round mags and in my opinion I would stick with the fixed 10 rounder. Much more reliable, just keep some stripper clips around and loading it is a breeze.
I would stay with the 10 round fixed. Just make sure you find good stripper clips. Some of the new ones don't work. Out of 20 I bought only 10 would work.
For those of you considering detacheable mags, remember that in order to legally use a detachable mag on an SKS you will fall under the purview of 922r and may need to replace some parts of your SKS with USA made parts in order to be in compliance.
I'm an believer in leaving it original. As previously mentioned there's the legal issues of altering an SKS and 922r. If you leave it original, you can't run afoul of the law. 10 round factory mag is the way to go.