I don't think you'll get a good answer here.
UM3x get volume out of either unit. Both units suffer the same exact roll off/suck out when driving balanced armature earphones.
One unit: Sony has a 'real' EQ and other sound operations.
One unit: iPod has very little hiss, while the other: Sony, hisses a lot.
One unit: iPod has gapless playback.
One unit: Sony has some dedicated buttons, but both are pretty silly in ditching nicely placed buttons.
The Sony is warmer, has a built-in EQ of about 1 decibel in the bass region - it is much warmer. If you like warm, go for the Sony. If you like neutral (and some call it boring), go for the iPod.
If you ask for SQ, people will say Sony, but they will always have said that because Sony have always played the 'SQ' card. The X sounds very nice if you can get by its hiss, but it is somewhat of a let down if you came from their and other MD players. The touch retrieves as much detail, but has a less dynamic sound due to in no small part, the EQ system which is hardwired into the Sony.
There is a lot of 'enjoyment' at the X thread which can point you to its good qualities.