That's like saying Blockland doesn't use Torque because Badspot modified it so much.
That was irrelevant, I'm talking about a physics engine, not a game engine. I think you misunderstood.
Half-Life runs on Source, and Source uses Havok as a base for physics. However, no one credits Havok because it has been so heavily modified that it is barely what it was.
I'm still deciding, and I'm not getting both because I am not made out of loving money, like almost everyone on these forums.
If you want me to, I can demo both games for you on a live stream, I think.