The only difference is the quality.
- Normal dungeons drop in blue quality, no monster head.
- Veteran dungeons drop in purple quality, guaranteed monster head.
Other than that the trait and item pool is exactly the same though I'm not that sure that it's actually sure that drop rate is 12.5%. I've been getting a lot of impenetrable on all gear pieces, including monster heads. Actually 2-3 days ago all monster heads I got were impenetrable. I suspect ZoS is manipulating the trait pool between incremental updates. That would make sense actually - because impenetrable was not included in the trait pool for some months, now has increased drop rate for a time to compensate. Has anyone else getting a lot more impenetrable lately?