In my opinion both. If you are on antidepressants and taking DXM you can cause serotonin syndrome.
Benzodiazepine overdoses are more likely to be fatal. But so can dxm. Depends on what benzos you are taking aswell.
I.e 5mg valium vs 300mg dxm. The dxm will be more life threatening and dangerous.
But taking 10mg xanax (or 50mg diazepam) vs 300mg dxm. The benos will be much more dangerous
I found dxm to make my anxiety worse personally.