If it was a Deception check directly against another creature (for instance, lying to that person), I'd tend to have the defense be Insight. If however the Deception was towards a third party or thing and the opposing creature was trying to notice it, I'd defend with Deception. So for example, if a person was creating a forged document, the initial DEX (Deception) check would be to create the document itself... and then the second person who might look at it would roll a WIS (Deception) check to recognize it as a forgery.
I only use Insight when it is a deceptive attack directly against the person in question (being lied to, interacting with someone in disguise etc.) when the person gets to "cold read" the other person (noticing fidgeting, profuse sweating, stammering, facial tics etc.) If the person is looking at some inanimate object or thing where no clues such as that would show up, I use their own Deception to figure it out.
