I wouldn't go for anything that doesn't connect via an optical output or 7.1 audio jacks. If it's connecting via usb then it is just a virtual surround that is using digital surround decoding. There are only a select few headsets that actually have true 7.1 connections for soundcards. I would recommend the Razer Tiamat 7.1 as they are the least expensive in my opinion and you get more out of it. Each driver can be adjusted and speakers systems can be hooked up to a pass through so they can still easily be used. Anything without actual surround jacks or optical digital connections won't be true surround.
After using my Roccat Kave 5.1 for a week, I find them very comfortable, very well built and verily formidable in gaming. They are good for movies, but cannot beat a 20$ Pioneer stereo headset in terms of pure sound quality when it comes to music. That is to be expected, considering it's a 5.1 headset, and considering that, the Roccat Kave is very good, much better than I expected. You don't even need much adjusting with the equalizer, if any adjusting at all.
Note that Roccat has just released the newer version, which is a bit improved but mostly the same.
I was thinking the same thing a few weeks ago.
it will come down to what sort of gamplay you are after eg. bf4 cod mmorpg or moba as an example.
I ended going with the Kraken 7.1 because i am more of a MOBA gamer.
The 7.1 virtual sound still works really well with BF4 but not as crisp as the a50.
I find the kraken to be the most confortable headset on the market and pretty light. this was a big selling point for me.
Astros can be more heavy and can pinch a bit afer long periods of gameplay.
It will come down to ur gaming preference. Both are good headsets. Choose for the type of gamplay ur after.