Given the choice, I would take the DT880 every time. The HD598 is a good headphone, but frankly, I would prefer to have a headphone that can perform low frequencies as well as it's other frequencies. The DT880 does that. The HD598, at least to me, lacks in the low frequencies.
Another player to add to this line up is the Hifiman HE-300. It will be $150 from Head Direct from Friday to Monday.