First off, the headset itself. The Astro A40s are very comfortable, they have a nice design that encompasses my very large ears, which makes it very good for extended play. The headset feels well built, and I like how the mic is detachable. In comparison, The SteelSeries headset is significantly lighter, so it rests better on your head, and I am personally a fan of the leather ear cups, which is merely a personal preference. However, they do feel more cheaply made, but not to the point where it would be a cause for concern.
The microphones are very comparable; Both are very clear and crisp, so you won't have anyone complaining at you. HOWEVER, the astro a40 2013 edition features a new mixamp, that has quite a few problems already. The mixamp has noise cancelling that I would consider too harsh, and sometimes cuts off sound before you're even done with a sentence. The mic used to be really low volume until they released the firmware update, but even then, the mixamp still dilutes the clearness of the mic, I find. Hopefully another firmware update will fix this, because by its own the mic is fantastic.
At this point, I was leaning towards the SteelSeries and was getting ready to return the Astros, but then I did a sound comparison test and that's where the Astro blows SteelSeries out of the water. The audio coming out of the A40s is leagues ahead of the SteelSeries headset, and with multiple EQ options available on the mixamp, I think most would agree. The Siberia v2 seems to scoop the mids out to try to provide a clear sound, but the Astro has the full spectrum and still sounds much better than the SteelSeries.
In conclusion, these are both fantastic headsets; The SteelSeries are very lightweight, very comfortable, and has a great microphone, at the loss of audio quality. The Astros, however, have a slightly heavier feel when wearing them, a few microphone issues I wish they had fixed on the mixamp before they even released the new edition, but much, MUCH better audio quality.