i own around 4 of them before. d303 has the best phone out. I love its detailed and warmed sound signature. I found the sound quality beat all the recently mp3 players as of 5/11/201. It also has very good line out but d555's line out is better than d303. the optical out is a plus if you use it to sync with your mini disc.
d303 has most common issue with the RF-410CH spindle motor and you often see skipping problem with that. in most case, if you hold the discman vertically it will play without skipping.