Like Fcvapor05 said, why would you care. What alloy they are made out of is much less relevant than how well they are made, how well they are heat treated, what is their history as far as durability and usability, and how is their warranty. And, finally, what is their price, and is the quality worth that.
For me, Snap-on chrome 1/4" drive regular and swivel sockets and chrome 3/8" drive swivels are so much better than others that the are worth buying. I don't think their impact sockets are better than reputable brands like Mac, Williams, Proto and Wright. I don't think that in 1/2" drive it matters as much, I'd take Proto, Wright or SK for a price savings instead.
If you will use them hard and professionally, I'd invest in quality tools instead of HF or other cheap import sockets. The reliability, better usability, and warranty will make it worthwhile, as will the intangible pleasure of just being able to use better tools. But, for sockets, if you're looking at costs, consider Proto, Williams (USA only), Wright, and SK sockets as a viable alternative.
The primary advantages of Snap-on are that they're marginally better than the others (that point is disputed by many) and they have a great warranty, with to your door service if you're on their route. If the warranty and service isn't readily accessible to you, you're paying a lot more for a small increase in quality (when compared to Proto, Williams, SK or Wright). Whether that is worth it to you is an open question that only you can answer.