Substantial means that the thing has substance, and implies that it’s large, solid, ample, strong, etc. A substantial thing is tangible or real.
Substantive means actual or essential, or having importance and effect, but is more abstract than “substantial” and does not carry as clear an implication of solidity, tangibility, largeness or strength.

Use “substantial” if you’re referring to a physical thing. Arguments can be substantive; houses can be substantial.