1. both 555 and 595 got 50 ohm impedance so both are good for listening connected to your laptop/ipod/mp3 player.
2. just checked now and price of 595s is actually lower than 555 on amazon.
3. both of them are very similar in size, so they wont be very good for portable use. on the other hand if you dont mind to look a bit strange its up to you
4. performance wise 595 are a bit more neutral (similar with hd 600 and 650) 555 are a bit more lively.
im sure u will be very happy with either of them, both are fantastic headphones.
