Up to you! The Sansa would be more portable, and you can put rockbox on it for EQ.
But then, the S3 is more handy, but adding a DAC/AMP would make it bulky.
If you're hearing noise floor with the S3, maybe look up some of the audio related kernal upgrades. Unless it's just an output impedance issue.
I'm not very good with my knowledge about output impedance and dac/amp specs can you help me figure this out? My phone is rooted.