For me, I am in the passive camp (my thoughts are in this thread on my Westone monitors). Custom in-ear passive headphones win on all fronts, as passive reduction is vastly superior to active reduction (18dB with Bose QuietComfort closed-can versus 25-27 with custom thermoplastic in-ear monitors). Non-custom in ear can also be superior to active reduction, but it is dependent on construction.

Combining the two may offer additional protection in a non-custom bud, but I'm not sure of how much additional protection it offers. Additionally, I would wonder about how well it controls cabin noise beyond air/engine noise -- namely people talking, PA announcements, in-flight meal trolleys, and so on.

I work in heavy manufacturing and can attest that active reduction is generally not acceptable as a permissible exposure limit control (at least that is why our industrial hygiene folks told me at a previous company). Given that I'm trying to protect my hearing, I'm comfortable with their advice.

Cheers.