I think what reward you get depends on what attempt you successfully decrypt. There seems to be three awards (Normandy Avatar Item, T-Shirt, Demo code) When I won the avatar item I got it on my first attempt, when I got the demo I got it on my third attempt.
Well, the post above indicates that's not the case.
Anybody feel like doing Exterminatus? I've got quite a grind left for 40 000 kills, played through campaign twice so let's say 5000 done. Loving the game so far, and I'm even not a fan of this series. Makes me consider giving the previous RTS ones a chance.