I hate to sound like a broken record, but I must say again that Cai Li Zhang is a fantastic singer who unfortunately will not get the chance to cut a solo album bcoz he just does not have the looks

As for chuang zuo, does it mean that you know how to play the guitar or write a few songs mean you are a chuang zuo singer? Then Andy Lau is also a chuang zuo singer, coz he writes the lyrics of so many of his songs.
Back to the topic : Stefanie Sun's voice is definitely much better than Stella's. Stella's live is pretty weak, as I had heard on a few occasions. Stella was INITIALLY discovered for her resemblance to Ryoko Hirosue, not because of her "talents". But the producers later discovered that she actually knows how to play the guitar and had composed a few compositions, so they quickly pounced on the opportunity to launch her as a "chuang zuo" singer. Now, that's some goss bits for you.
But frankly, we shall see how Stella fares in another aspect now that she is also acting in ou xiang ju. Maybe she will find herself "another piece of the sky" in acting...