According to www.dfwdepot.com
, the DVD-121S has the following improvements over the DVD-120S:
* Newly developed loading mechanism for increased reliability
* New construction transverse mechanism, base chassis, disc clamper and thicker metal cover all contribute to lowering the acoustic noise and vibration levels
* The drive length has been reduced to fall in line with the new industry standard 198mm
Based on the specs the performance is the same, but it's better made, making it quieter and more reliable. It may also support more disc formats. There are so many I didn't try to compare them all.