Hey there BenEzz!
I have both the Astro A40 and the Kraken Pro.
The Kraken Pro or 7.1 will be a more of a bass heavy headset. So if you're planning to play CoD or Battlefield it'll be harder for you to pinpoint the exact footsteps with a bass heavy headset. Also, the 7.1 version is a USB only headset.
Now about the Astro A40.
I just bought myself the bundle with the mixamp anddddddddddd I have to say this headset blows the Kraken out of the water. The quality of the sound is a lot clearer with a punchy bass and isn't bloated like the Krakens. However in terms of comfort I have to say the A40's does it for me. There is a whole lot LESS clamp with the Astros vs having to readjust the Krakens every other hour.
However it is solely up to you to which headset suits your needs. If you need a universal headset for Pc, Xbox, Ps, etc get the Astros. If you only play Pc get the Krakens. Also the price may factor into your decision, Kraken 7.1 is a LOT cheaper than the A40 and unless you can get a Astro with a Mixamp for 200 or less I wouldn't say they are worth it.