dt990 is cheaper in europe than the hd598 and given you say you already have an amplifier (you will want an actual headphone amplifier for the dt990, not just some crap onboard soundcard that states it can handle 600ohm cans so keep that in mind here as you did not specify what you have) then the dt990 maybe the better buy but it really depends on what kind of audio signature you want.
the dt990 is more v-shaped, bassy, sharp and detailed while the hd598 is more flat, slow and mellow. both are good depending on your preferences though europe pricing on sennheisers last i checked was much higher than beyer. if you need an amp for the dt990 though that will increase the cost.