I'd take the b18. You can get em for $1000-1500 complete swap with trans, axles, etc if you shop around. Upgrades will net you a little more power vs the d16, and it will make a significant amount more torque because of the extra displacement.
But the d16 is a little bad ass in itself. Weighs 100 pounds less than the b18, too. And you can buy the little bastards for ~$300