Get the J3 for the slightly better batter life, just a bit, and the microSD expansion slot. Otherwise the S9 is just as good, and can be had for a few $$$ less. They're basically both the same player w/different features, as the J3 is a SLIGHT upgrade from the S9.
Either way you go, you'll get an excellent player w/a great screen, the best battery life, huge codec support, a name synonymous w/stellar SQ, and an EQ and sound enhancements that's only rivaled by Samsung. But Samsung doesn't provide the support or the great firmware updates the the Cowon community gets when the S9 or J3 are concerned.
All in all - choose one or the other, you'll be darn happy with your decision.