as far as i know theyre pretty much the same engine except the b20 has a longer stroke, so ya if you supercharge it, it will make more power than an ls. www.jacksonracing.com has dyno sheets for supercharged ls and b20s if you wanna get a better idea.

the b20 makes about 10% more torque all the way to redline so i would say if the b18 is more than 10% cheaper than the b20, than go with it.