Here’s how my friend, Paul Batres (manager of San Jose Pro Drum) compares the two heads (and rest assured, he’s not trying to influence your buying decisions):
The SSTs have a more defined attack and a short sustain.
The Onyx has a darker tone, short sustain, but not as defined of an attack. And because of the black coating it adds to the darker tone and adds extended durability to the head.
I have personally used both, and found the SST best fits my playing style. Whether you choose the Onyx 2-ply or the EC2 SST for your batter head, I would recommend using either the Genera Resonant, or the G-1 Clear for the bottoms.