QUOTE(Jojo @ Sep 28 2007, 01:34)

I will mainly use them on my iPod and maybe soundcard.
In that case, you should really go by wear comfort. Those devices aren't known for stunning high-fidelity output.
I was facing that decision some while ago when looking for some good open phones to replace my HD540s (way better than the new models), and I found both Sennheisers to be very uncomfortable because the hard circumaural cushions rested on my jawbone. Also, Sennheisers are all bass-oriented but still are good open headphones, don't get me wrong there.
I finally went overboard and got me a pair of AKG K271s. Those are closed, but very, very comfortable. They should be somewhere between the 555 and the 595 price-wise and are far less bass-heavy -- I'd go so far and call them weak --, but that's what you get for monitor characteristics. They are also prone to solid-borne noise from the cable and the headband, but unless you're doing sports, they're fine even over some hours.
Perhaps have a look at them, in my opinion they are well worth consideration. The price is in the same range, and they will happily reproduce whatever you throw at them.