As far as sound quality goes, I can't imagine either being better than the other. So based off that, you're paying for design and how the item functions.
The only difference I can see is the Earforce DSS allows you to adjust your bass boost whereas the astros mixamp probably has some already applied that you can't adjust. This is nifty seeing as how you can decide what games should have/need bass boost and what games sound perfectly fine clean.
I bought the Earforce DSS nearly a year ago and haven't played a game without them since. My first online game of MW2(footstep frags ftw) with these pretty much made me a proud owner. Just make sure you have a solid pair of headphones.