Difference (from what I know at least):
blow-off vents into the atmosphere whereas the by-pass vents back into the intake track after the maf.
We should use the by-pass because the the air is already measured by the MAF; so if you just vent the air into the atmosphere, it won't run as well. The by-pass vents back into the intake after the MAF, so it isn't measured again, but still goes through the enigne.
I think with systems that use MAP sensors, you use the blow-off because it's not measuring the mass of air, but just the manifold pressure.
'96 SE-R.... a few tweaks here and there