Are you going to be listening to music through earbuds/headphones, or using a vehicles audio system?
What environment will you be in most of the time (quiet office vs. plane)?

Lots of different options--the difference will come down to the headphones/speakers you are using since 99% of the artifacts/distortion comes from the analog part of the equation....

Just remember that there is a standards "war" and if you buy now you are locked in to the DRM technology supported by the vendor/device (ie Apple=iTunes). Not a fan of any of them right now.

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'08 335xi - Black/Black - 6-speed manual
'07 Mustang GT/CS - Red/Black - Nuthin' Beats a V8
'04 X5 4.4i Silver Ext. / Gray Int. / Loaded (Gone but not forgotten)
'05 Mercury Montego (Wife's Car)
'03 H-D V-Rod -- Custom Paint, Wheels, Seat, Exhaust, blah, blah, blah
'07 BMW R1200S Red/Silver -- Light 'n Fast