which headphone sounds like you would enjoy it better? i would worry less about your soundblaster z and more about what you like the sound of. the akg cans are very bright so unless you like bright cans you might get fatigued by them easily. good cans for sure, if you like that type of audio style.
again, i'd like to know why you are concerned about the Z. for either of the above headphones the z's amp is a bit too strong and you might be running windows volume levels lower than average. i would say that the akg's are going to be just a bit harder to drive than the senns though.