Internally, there isn't a difference. The two versions have the same dimensions for the expansion chamber and number of vents.

Externally, the BC1.5 has a stand-off hood to increase its length, primarily for those folks with 14.5" bbls who need to get to 16.1" bbl OAL for legal reasons.

The BC1.5 comes pre-drilled to aid with pinning and welding as is often required for those 14.5 bbls, as well.

On an 18" bbl, the BC1.0 should be quite acceptable. That is not to say the BC1.5 won't work with different barrel lengths -- it's just the BC1.5 was designed specifically with the 14.5" bbl in mind.

Hope that helps!

Best,
Alan