The B18B's cylinder walls are a little more re-inforced than the B20B. Don't hesitate on what you hear about the B20B's cylinder walls being "weak" as this applies to any numb nut who doesnt drive his car correctly and totally loves to over-rev and destroy his motor.
But never the less, the B20B's bottom block has more potential if built correctly. For example, running 10 or more psi on a stock block is just insane. Both blocks will hold boost well with mild internal upgrades, but B20B is the better deal, more low end torque and peak torque, big bore etc.