i3 is better all around, though generally a little more expensive. It's basically a class above the poor old Turion. The Pentium Dual cores are closer in comparison to the Turion X2. Turions are good enough for most users, but even though they are generally clocked higher than the i3s the i3s preform better.
By the way, Fusion parts are starting to ship. The GPU on the fusion has twice the shader power than the chipset the Turion X2 is generally paired with (80 vs 40), and is much more power efficient. Of course it's mostly a low power solution for thin laptops/netbooks with long battery times.