Compression ratio is just what it sounds like... the amount (in a ratio) your engine compresses the intake charge when the piston comes up.

So, for example, a 10:1 compression ratio would compress the air to 1/10 its volume.

Between those engines... the b18c1 would definately come out ahead, but one of the points of using the b20 is its lower compression ratio (lower compression ratios are more boost-friendly), so you'd be able to nudge the boost higher on the b20.

After taking that into consideration... it would be very close.