The X3. Triple cores are extremely good for gaming right now. Also, both could potentially be unlocked to a quad core, but the problem is that the dual core would be less likely to be stable. In addition, if the unlocked cores aren't stable, the dual core would still be a dual core. That would greatly hamper performance.
Not only is the X3 technically better for gaming, but it's typically lower cost means you can spend a little more money on the GPU, which is vastly more important than the CPU anyway.